They'd figure out how to upgrade eventually, but if a new player were to download ET, install it, and go to the server list, they'd most likely be confused as to what the problem is.
Luckily with new games there's an auto-update feature eliminating the problem, and the age old discussion over players staying in older versions.
I still personally don't see a problem with players staying in an older version of Enemy Territory if they enjoy doing so. I play 2.55 and 2.6b Enemy Territory. As long as I enjoy playing it, I'm happy.
I do understand you point with RtCW, however. The lack of PunkBuster in 1.0 is a major downside and hacking there is, obviously, going to be much worse than in a version which has anti-cheat software included in it.