Suggestions for best course of action to convince parents that cutting the cord is going to save them quite a bit of money and they can still watch all their programs?

Set up the system for them and let it do the talking I guess?
They can have both until they realize they don't need both.
- This is the best option. Put the system in and go over a lot and show them that ALL of their shows they want are available. Then show what it costs. Pay for it yourself as a gift to them. Once they see the bill (if it's cheaper) and once they see ALL of their shows are available, that should convince them.
  
  I did this almost completely except for PBS and Peacock because they * have everything else already *. Anything that’s different and they get deflated because they just want what they are used to

My grandfather had a wood burning stove as his only source of heat for YEARS. Refused to swap to gas.
One day, we distracted him with some friends and removed his stove literally behind his back. The next day, he was complaining he hadn't done it 40 years ago.
So maybe just do it when they're distracted.

Alternatively, if they are happy with their pro-grums, familiar with the interface, and not getting gouged worse than the next cable subscriber — consider letting it rest.
From things like being able to punch in a channel number to having the cable company to call for issues, folks in that generation might really prefer paying.
I’m basing this on my years of experience with a now 60-something and now 80-something. YMMV
- This. At some point I bet cable tv will be a lot cheaper, and you suffer ads, which is how it always should have been. 🤣

At the end of the day if they can afford it and don't want to change you've got to ask yourself if it really matters?
I know it's silly to pay over the odds for something but it's up to them really.

If your parents are struggling to make ends meet at 77 then continue pressing the point and let them know why.
If they’re not let to go.
You’re good with tech and know they can save some money. If they don’t care about tech, and aren’t hurting for money, you’re just being annoying.
Had the same convo with my mom. That’s where the annoying line came from! She likes what she has and can afford it.
Now my sister ima boot in the head because she cancelled my mom’s landline for a cheaper cell plan. So now, oh…about 80% of the time I try to call mom her phone rings right into voicemail because she forgot to plug it in!

They are just being lazy and don't want to learn something new. Just let them do what they want. They're adults

Don't do it. They'll blame you for every perceived problem they have (most of which will not actually be problem).
It's expensive to refuse to learn new things.

How many good shows are no longer available, or required a more expensive package to keep? If that hasn't happened yet, it will soon.
My mom constantly complained about her favorite shows disappearing, so I bought her a ROKU and plugged it into her TV. She canceled her cable two months later.

Literally cut the cord for them. Seriously though. If they are happy with it and can afford it, I'd just leave it alone

- I think you're underestimating how resistant to charge old people can be.
  
  “Simply present the relevant evidence. Surely they’ll change then”
- I did up a very easy to understand spreadsheet.

Why do you care what they use? You are factually correct, but everyone knows that end user experience trumps all, and familiarity is a huge factor in that.
Let Meemaw have her picture box the way she likes it.

Maybe try to ween them off cable by cutting down to ultra basic cable. Still have access to local news.

Is something like Plex an option? Get a hard drive, set up a request system (Ombi), then a little app on their phone can take requests and download the shows they feel like they're missing.
Done right it's a one time set up, and an up front payment but over the course of a year I feel like you're saving the cost of either DirectTV or the streaming services.
Also, you can tell them that DirecTV simply licenses shows the way streaming services do which is why they won't lose any content.
DirecTV also may something they prefer simply for what they're used to. At 7pm the same show comes on every day. If that's a part of it, my previous suggestion of a server for media is able to put together TV channels using something like DizqueTV. I understand that it's a bit of effort, though.

If they have to have a lot of channels then $120 isn't the worst price (I have relatives who pay twice as much as that a month for cable) though you could perhaps try and check into whether they could switch to a streaming linear TV service like DirecTV Stream with one of their lower tier packages to save some money while retaining a cable-like experience (there's also Sling, Hulu+Live TV, YoutubeTV, FUBO, etc, many of which have packages with many of the top channels for $60-$80/month).
Fact is to save money you need to be willing to give things up. If you're moving from a premium cable package with a ton of channels to a few streaming services you're going to lose things and potentially a lot of things. You're going to lose access to live news channels, you're going to lose access to specific programs on some networks that don't have streaming service equivalents (I know for one older person I knew the fact they couldn't get and watch Lifetime and Hallmark movies within any reasonable time-frame of their premier meant they were not interested in looking into streaming any further to replace their cable).
More than that though most old people hate change, they were used to a certain way of things and they're afraid and perhaps get confused or frustrated with this new way of doing things. It's simply more comfortable for them to use the old satellite system they're used to and its interface and way of changing channels than doing something new where they have to think of how to do something or get frustrated or ask for help. Which is why I do think trying a streaming cable replacement like those I mentioned might be your best bet. It would still save some money.
- They actually won’t lose access to anything. I made sure of that before I began this initiative.

The simple truth is they probably are going to be missing out on those programs. Certainly that's my experience. However I think it's generally a good thing.

For my grandfather... The issue wasn't the shows, but he specifically wants a few news programs and will not under any circumstances go without them.
This was a problem for even going to Internet based streaming options because he just will not accept anything without those shows for more than a few months.
Meanwhile he also complains he doesn't have enough to watch and says he can't afford it (he can, he just doesn't like what it cost)... But those dang news channels... and just his outlook on TV in general.

- If they use the TV box all the time, why do they have those apps?
  Exactly. (They use both equally)
  They are on a fixed income, and I would rather see them get a nice new mattress for my mom’s back pain, so she’s not in the living room every night sleeping in the recliner.