IS:
I’m not talking about someone who is mature and takes his learning seriously, learning according to what he is able.
In the kollelim that I have been to, the ones that aren’t sitting and learning are the ones that don’t take learning seriously. Yes, some of them will mature with age. But I think where it is clear that someone in kollel will learn at least a little bit and not clear that leaving the kollel will cause someone to learn it is preferable to go with what is sure.
Mature people who do take learning seriously but can’t put in the hours on a daily basis, are already not sitting in kollel all day (v’horaya you and your chavrusas), which makes them irrelevant to this discussion.