Why has Russia decided to stop the F-57 production ?
The Sukhoi Su-57 is the 5th generation, stealthy, single-seat, twin-engine jet multirole fighter aircraft designed for air superiority and ground attack operations. It is the successor of Su 35 and Russia’s answer to American F 35 Lightning II and F 22 Raptor.
Russian explanation
If you set the propaganda aside and try to look at it without bias, you'll see that the main reason for not going into large-scale Su-57 production is the same as why US stopped production of F-22 at 195 units instead of planned 750. They are both expensive and there is no real need for them. 20 years after start of production and $70B later, F-22 does not have a SINGLE kill under its belt. This is not a critique of F-22, simply a statement of the fact. Given its unique combination of cost, export ban and lack of worthy targets, it will likely go to a retirement without single kill. F-35 has much better chances to see some action.