Simulators are indeed few and far between, but on the other hand there are +400 frames standing idle on the ground, why not bring them to use? Sure, it costs a hell of a lot more than using a sim, but, using them will reduce the time needed to get all crews trained.
So, how about we have Boeing trainers train the airline TRI/TREs in the real aircraft, and then have the TRI/TREs train the TCs in their grounded fleet, who in turn may train the line pilots, either in real aircraft or in simulators as available.
Such an approach would also have a large and positive effect on regaining public/airline/pilot trust. But such pragmatism is of course far less appealing to your average bean counting CEO than a 1-hour iPad session.