I think it is multi factorial.
Yes, training classes are backed up and that is partially because of the majors increased hiring that disproportionately benefited applicants who were LCAs and trainers. That left somewhat crippled training departments that would have had some difficulty with the old attrition rates but was totally unprepared for the number of people they would have to train to replace the suddenly increased attrition, even had these new hires met the historical levels of experience that the training syllabus had been designed for.
Except most of the new hires didn't meet those historical standards. In order to fill the increased attrition, people were being hired that two or three years ago woukd not have been competitive applicants, and being pushed into training programs that required a pace that many were unable to sustain. To keep attrition down, extra training was necessary, which exacerbated the problem of lack of experienced trainers. Despite an overall shortage of sims - due to this high attrition - more sims had to be added to the training footprint, which slowed the process further. And it was aggravated again immediately after the summer flying when - again - the majors went on another hiring spree that again disproportionately affected their training personnel.
And that is a huge chunk of the current problem but for the new guy who would have probably been a quick hire (with bonus) two years ago there is another problem.
A finite but significant number of people who sort of fell out of the queu during the "lost decade" when there was relatively little flying and gave up to go into non-flying careers are coming back, fueled by the bonuses, the higher salary, and the anticipation of all the retirements at the majors. And compared to someone just starting who barely meets the ATP requirements, these old fogies with maybe 500 hours of 121 or 135 flying already behind them (and likely a type rating) in addition to that 1500 hours of mainly CFI time, are a considerably better training risk than the younger guy who (barely) meets ATP mins. And that means they are less likely to require any sort of remedial training and avoidably use up those precious FPT and SIM and instructor hours.
Now there are not a huge number of these guys, and most of those who are going to come back very likely will make the move by this summer, but I think it is quite likely that a number of regional's are going to take the opportunity to increase their hiring standards or at least preferentially hire these retreads, to the detriment of the 1000 hour RATP guy who would have been snapped up a year or two ago.