The thing to remember with these examples is that those companies would have royally fucked up their purchases. Big companies always impose a culture and a mindset.
AT&T would definately have crushed the internet with a monopoly - we would have had to use AT&T approved internet devices, and they would have brought long distance type charges to it. Oh so your email is going overseas? That’s an extra 10c.
Same with Google and Netflix. They were all able to continue with the founders vision and create something special.
That one really wasn't as obvious at the time. Netflix was in huge debt and hadn't really built their streaming platform yet. In fact streaming was barely possible. Blockbuster should have been able to out-compete Netflix at both dvd by mail and streaming but they screwed it up. Netflix won, but may now end up getting killed by the big studios.
It never is, that's how the investment market works. Blockbuster thought it was still ludicrously high for an ailing niche competitor. That's arguable, but I don't think anyone could guarantee Netflix was actually going to achieve their (at the time very sci-fi) vision.