When i see how many people are having fun with "extended" tracks i would say PD would be stupid to stop this. It's not only about extreme tracks like rollercoaster or k1, all tracks which are made with the "extended" CM are much better because you have much more possibilities to create more "organic" bends and curves. And the free placement of S/F straight gives us much more variation to do tracks in the interesting areas of a theme.
If PD wants to stop this (what should be very easy for them!) my interest playing GT6 will be about 50% less. I don't think they are going to build new tracks or cars for GT6 since they should be all working on GT Sports. So which constantly new excitements will be left if not racing "extended" usermade tracks? The "normal" CM tracks are somehow all the same after a while. Only "extended" tracks are real new fun today, and the players working on this tracks is what makes GT6 interesting again day by day... and for free! I guess PD knows this and stopping such a good feature they pay nothing for should be not a good idea