What can be done to prevent this?
Why do you think this should be prevented? "Accept this life-cycle as a reality" seems like a reasonable approach. The "old members" can keep their identity if it's really tied to hardcore-X. Or they can sit on their porches and grumble about kids these days :-)
Why do you think this should be prevented?
It's a waste of resources.
For example, if the LessWrong website becomes boring and people move somewhere else, they will again have to design the web page, implement technical features, etc. Even more resources lost if the same thing would happen with MIRI or CFAR. Not only the money and work would be lost, but also prestige and contacts; some useful people would probably be left behind...
There are more options than just "go away a start again" or "grumble". But these two are the easiest options, which is why people have to choose between them if they are not strategic.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.