If they want heavy fighters then why order any MiG-35s.... and why is everyone so excited about this LTS design?
Probably because they were ordered to. The USAF is forced to buy stuff all the time. In terms of the LTS design, the design landscape has changed with the advent of stelath. With all the focus on radar cross section, one sure way to reduce it is to make the airplane smaller!!! Also tech has allowed more capability in smaller platforms. Notice this plane is actually quite long, it will probably have quite a bit of internal fuel on board and this suits the RuAF doctrine. I am not sure if the MIG-29 is much cheaper to operate. After all Malaysia has parked their MIG-29s and operates their SU-30. But that is probably more to do with the need to support the anti-shipping role of the SU-30, and their large inventory of KH-31s. LTS is a shiny new object, every air force likes new toys.