Monero and Ethereum have very visible heads within the project... this is the problem of being the leader, like it or not your word has a massive impact.
If fluffypony or Vitalik make a tweet saying "we need to support X fork because of Y" everyone will support X fork, but still, we've already had ETC being a viable altcoin which should have died a long time ago as (wrongly) predicted by Vitalik.
Bitcoin is just on another level of decentralization. No one in Bitcoin has the influence that fluffypony or Vitalik have, or any other founder. Cryptocurrencies just don't work without an anonymous creator that eventually disappears.
Ethereum makes regular hard forks to upgrade the network. When a coin is built from the ground up to expect such things it's not too bad to get people on-board. But yes, if people continue to mine the old chain with old software, it will still exist.
Well we'll see what happens when they switch to PoS... I want to see that one. I think having an ETH-PoW/ETH-PoS situation is a realistic one, they are basically firing all miners, and they may troll them creating another ETC situation (this time, ETH-PoW).