The problem is that Sense isn't just a launcher. It is integrated into the OS. If it was just a launcher like the others then the updates would come faster. With Sense there are a lot more customizations behind the scenes (no pun intended) than what you think.
Does it shut down or go away when you use another launcher or home? No, but that is because it can't. The modified apps like people, browser, calendar, dialer, etc. all require Sense to be running in the background so that those aspects of the UI can continue to work.
What would be nice, is if HTC did what Google plans to do with Froyo and Gingerbread. HTC could have Sense divided into different aspects of the UI on the market. They would only be available to HTC customers, obviously, and could be downloaded in pieces when updates are completed. That would solve fragmentation as well as EOL issues. People could update one day and then a week later you would get an update for the dialer or the calendar. This would also make patches much easier as well.
Personally, I really like Sense and I'm not sure if I will buy an Android without it. The only thing that would stop me is if a really amazing feature came out that was going to take forever to be integrated into Sense or couldn't be. Then I would probably buy a vanilla Android and Root it with a Sense type ROM anyway.