While this does have some logic - air forces typically have two approaches towards purchasing aircraft. One is to purchase a smaller number of highly advanced sophisticated aircraft (like the PAF F-16s or IAF Su-30MKIs) and also to have a 'force-multiplier' option by having a larger number of relatively simpler aircraft (like the PAF A-5c Fantan, F-7P/PG or IAF Mig-21s).
In case of Bangladesh, it would be Mig-29 (being upgraded to SMT version) for former role and F-7BG for the latter role.
I don't think you could use trainer aircraft (L-39, K-8, Yak-130, Alenia M-346) as force-multipliers because their flight envelope doesn't allow it. They may substitute nicely as COIN (counter insurgency) use however.
Older F-7PGs may be utilized as cannibalization sources to keep our F-7BGs flying. We do need a large number of F-7BGs to keep the aircraft count up. In that case - local capability have to be upgraded to overhaul Wopen turbojets used in these machines. And like the OP mentioned, F-7M's can be sold off to airforces still flying them.
I see A-5c's to be not as useful because these were already based on 60's technology (F-6 or Mig-19) and leave a lot to be desired aerodynamically.