Obviously you’ll be able to run older versions of the framework so long as the underlying OS supports it but depending what type of environment you’re in this means you’re likely going to have to update perfectly working applications every two years.
Do I think that it will take a massive amount of work to update from dotnet 6 to 8, or 8 to 10 in a few years? Probably not, but that could change at any time depending on the whims of Microsoft on that particular day.
I get that the current technical zeitgeist is that if you’re not releasing a new version every other day you’re falling behind but five years seems a bit more reasonable to me.