Indeed, why update the old games if people don't play them anymore? I agree. It's a waste of resources. (Not saying nobody plays Storm anymore by the way, just using this statement as an example.) However, like I said before, Nadeo made a choice of "unifying" all the games into one (ManiaPlanet) which requires them to maintain all games. (Or at least make sure that all games are working fine after every update.)
This is why I think Turbo is in a really good place - it hasn't received updates in a very long time, so the game won't ever break unless Nadeo happens to take the servers offline. If Turbo was part of ManiaPlanet, Nadeo would be forced to maintain Turbo until the end of ManiaPlanet's existance.
This ties in to what I wrote above. This is not possible with the ManiaPlanet model, these things need to happen at the same time.