I gave the Su-30 as an example. The Yak-130 is capable of simulating the likes of F-16, F-15 and even the F-35.
Ask yourself, can you fly a 747 after just reading a manual?
SAMs are good, but they have limitations. Any air force would need trainers. Heck, even the USAF use simple trainers. Does that mean they are vulnerable from the Russians? A trainer and fighter are not the same. Trainers are vital to make any air force sustainable.
Another thing is that those Yaks are not only meant to replace BAF's aging fleet of trainers, but also replace the aging A-5's. This partly explains why BAF ordered in such a significant quantity.
Those A-5's were meant for CAS, and so would the Yaks. However, I never really understood BAF's need for CAS aircraft given their current inventory and operational setup.
For CAS aircraft to be able to perform their operations, the air force must be able to achieve total air superiority over the enemy. Otherwise, they are toast. We'd never see A-10's being sent out first in a traditional war theater. Those A-5's were bought back in the 80's during Ershad-era if I'm not mistaken. So that leaves me with some question marks.
As far as India goes, I don't think they would simply jump in and start randomly bombing Bangladesh. Otherwise, all that money spent on Bollywood propaganda would go up in smoke. Why do you think there's all this tight-lipped conspiracy in light of the things going on in Bangladesh? It is specifically because of that vulnerability.
Bangladesh will never win a battle of attrition against India. The Israelis never fought that way against the Arabs during the Arab-Israeli wars.
S-300's and BM-30's would not pose any real threat to them other than a bunch of whiners.
For Bangladesh, there are "other" kinds of warfare. One just gotta be creative. One can actually contain them without firing a single bullet.