I don't think it's feasible to either support hundreds of different versions of C3 (depending on when various people's updates have expired) with essential fixes, nor to completely abandon users whose updates have expired (some things will simply have to be fixed, like when a new iOS update breaks something or other). There are other complications like the fact the free edition is kept up-to-date, so if you logged in to an account with expired updates, you'd have to somehow be downgraded to an older version, which is difficult, and breaks compatibility with any projects you were working on in the free edition.
Paying for a period of updates probably works if the software updates about once every six months. We, on the other hand, have done two updates this week already... and it's only Tuesday.