First of all GameVault was never Open Source.
Secondly this is not anything like that and I'm sad you didn't even read the full blog post. It even says it's not a shitty enterprise move.
It is just a way to give something back to people who would donate anyway. The plus features are designed to be bypassable with minimal effort if you don't want to pay for it. The extra features are not important to use the app as usual. Please read before you judge.
It is the old crack pipe but it already features way more than just visualizing your game collection and there are a bunch more features in the oven.
The 49€ a year have been selected very carefully and when comparing it to other subscriptions today, are actually pretty reasonably priced.
You don't need the subscription or it's features to use GameVault. It's designed to be totally optional and bypassable. If you enjoyed using it until now, nothing will change for you. See plus more like a "thank you" for a 5€ donation monthly instead of a real subscription service. You can compare it to stuff like patreon.
you do you, its just a warning because of potential malware, i'd personally rather self-compile than pirate something when i have the damn source-code.