If an addon dosen't work with the up-coming 2008 version, chances are that all the addon developer will have to do is re-compile the addon, a process which might only take a few miniutes (I think, I don't actualy know, though my programming experince leads me to think that). So the most popular addons will be made compatible within a few weeks, if not days, if not hours. On top of that, there's a public beta of the new version, so developers will be able to check their addons in a sort of pre-emptive strike. And remember, no-one's going to force anyone to update.