We recently ran a poll for team members to find out when they plan on getting the new Xbox One console. Only one member said they would be getting it at launch. The rest of the team that was planning to upgrade was split with half saying they would get the console in 3-6 months after launch and the other half 6 months to a year after launch.
This seems to be true for the at large Forza community as well. VM has announced that they will stay with FM4 for all their official series in 2014, running occasional test series on FM5. IFCA, as well, has indicated that they will continue to host race series on FM4. TORA, on the other hand, has said that all of their official 2014 series will be on FM5, while unofficial community series can continue on FM4.
Microsoft will continue to support the 360 for at least another year. New games will continue to be released for the 360 platform, most notably Project CARS which is scheduled for early 2014.
So, while those that migrate to the XB1 at launch will probably stop playing the 360, for those that don't migrate there should still be organized racing on the 360 for a while yet.