YouTube's stronghold is because of community, not technology. It's also been around for 14 years and has so much internet history baked into it that it's just hard to conceive it being dethroned.
As for SEGA, AOL, and IE, they never had any real stronghold (like a lock-in), they just had the users - those are easier to switch to something new. Games for example go stale, and you don't care that much for your older played games - you want the new shiny. A browser, you switch over, and you have everything you did before, including your bookmarks. AOL, had it's stronghold when the internet was 1/100 of what it is now.
I can see the internet going 100x larger now (except if it goes to 80 billion people out of the 8 billion on Earth).
right, Windows didn't lose their position because someone out-competed them in their area, but because a whole new category of device (smart phone) and a new category service (search) rose to prominence. (Also fears of legal problems tampered their normal behavior)