Nothing wrong with the Singel engine F-35 jet.I think russia has to build a decent engine in size and reliability before they could even consider their main plane to be a single engine.
Also I saw at key that you don't seem to realise that the f-35a is a SL 750kn with internal bombs and missiles airplane.
It has a dry cruise of M1.25 and restricted/limited to a max of M1.6 wet (not it's max potential speed), also whilst with internal bombs and missiles. I'll let you get the chart for Kn, alt and Mach numbers.
Its just that Russia is quite happy with their own twin engine design Su-34 for much of the same role, A2G. No doubt the Su-34 has far longer legs vs the F-35.
My point is, according to Russian Air doctrine, strategic and tactical role, they do not need new singel engine jet.
What is desired within USAF does not mean its desired in VVS and vica verca.
I don't see how people can claim otherwise.