Actually its different -
Role of MKI with BRAHMOS will be tactical and strategic one , it will give an unbreakable lead to india to strike as Nuclear option in air.
Bcoz only Su30 can go deep inside Pakistan or China and strike the strategic asset .
Thats why very limited nos of MKI are upgraded.
Mig29 role in IN will be protection of surface fleet for any hostile asset whether ship or aircraft.
And look at the capability of KH31 on Mig29 - Its much more faster than Brahmos at 3.6 MACH, its skimming nature makes it more worth as anti-ship platform on MIG29 plus you get that at a mere cost of Brahmos .
Brahmos gives us clear edge at cost and will only be as tactical asset to deliver precise Nuclear blow and give decisive Punch.
Once we get KH-59 improved with sonic speed , no need for modifying Mig29 to get Brahmos .
MKI are our Nuclear deterrent and strike platform , Mig29 are not for that role.
Secondly weapon integration takes time , sometime years and only Manufacturer do so . In case of Brahmos modification are to be done in Firecontrol radar , software , HMS , and FCS . When you release such heavy missile you need better ares-dynamic aircraft to give necessary kinematic force to missile , and when suddenly such weight goes down FCS has to adjust suddenly . It req lot of modifications and testing , so why do it when you wont be using this aircraft for that role.