My opinion is that they aren't going to offer the training until they have to.

There will be a backlog of pilots who either already hold an ATP or have at least taken the written, and those pilots will get picked up first. Then the airlines struggling to get applicants will have to start offering the course the soonest, then finally the 'better' regionals if they can't get applicants. And, of course, they won't get immediate approval from the FAA for those courses when they do decide to start offering them. Also, I speculate this may prompt airlines to bring back training contracts.

Bottom line: if you are going to have 1500TT in the next two years it's probably a good idea to get the written done now.