I don't know how easy it is to move from private jets to airlines, as private jets are maybe under the MTOW threshold that some airlines look for (not sure of the exact figure, 25t rings a bell).
Certainly in the UK it is not rare and far from it to join an airline operating Airbus/Boeing with as little as few hundred hours on pistons or turbo props King Air style.
All companies have different mini requirements but what is truly critical is how much and how quickly they need pilots. Right now the answers are... A lot and Yesterday.
But if they are going through a rapid expansion then they will favour guys with 3000 hours or so with some time on heavier equipment than a Seneca simply because the possibility and feasability to recruit Direct Entry Captains are the limit to their expansion/strategy. What I mean is that they want guys who can upgrade really quickly which translates into guys who meet nearly all the mini requirements to get through an upgrade course.
But having jet time on pretty much whatever is a big, big advantage. Nevermind the weight, specially if you are in your 20s.
Hardly no company expects a 25 years old to have 3000 on Jets over 20 Tonnes. Not that they don't exist but they are no necessarily the norm so airlines take a more practical approach towards they recruitment requirements.
In short and to answer your question, the 25 Tonnes thing you have in mind might be a requirements for one or more companies but no more than that. Requirements change and many have different ones.
I flew my first 115 Tonnes jet right after flying a chieftain... Until then I had never flown anything bigger than a 1900.
It wasn't that long ago and it was in the UK so here is hope for some...
As for now, well, I have left the airlines and enjoy flying Bizjets. Not for everybody but my kind of work. Like you though, assuming the job market remains good and talking about people getting on this scheme in their 20s, 30s, I too wonder what proportion will stick around for more than a couple of years.
Not necessarily because they don't like the job or the company but mostly because they will probably want to try their hands on the big stuffs.
Regards to all