If Soviets exists now, they will manufacture 1000s of jets today, USSR == 100x Russia, don't forget Russia equipped with large no.of mi31[heavy], su27[heavy] class jets[left overs].
Its true maintenance cost of jets are increased, yet it is low compared to non aircraft items - for ex. consider phones, 350 inr capital cost with 250inr monthly cost[old] vs 10k inr capital cost with 250inr monthly[now]. IAF holding mig21 is like a person using both feature phone & iphone . In simple su30mki is very expensive to maintain compared rest with low availability rate & sortie rate & need two pilots to fly and IAF need a low cost platform for low cost operations with high availability rate, like lca. Normally countries chose low[70%]-hi[30%] or medium[100%] in their AF, but IAF stuck with 270 mkis, forcing them to keep m2k, mig29, mig21 with expensive upgrades. Best solution for this issue is more local content in mki & 200 lca mk1 & 250 mk2. Mig21 is equipped with fcr, hmcs, r77, spj, ccm etc.
Mass manufacturing depends of industrial & manufacturing sector & economy - we are multiple decade behind chinese and cost of human resources is exploding. Right now we can only sustain 30 jets per year compared to 500 per year of chinese and they can easily increase further, but we can't, similar to US, French etc.
F35- its designed for low cost operations with high availability rate unlike other stealth jets & its operational cost is a higher due to over engg. and come down in future.