>>51691
If the Ukraine manages to prolong the war long enough there is a chance that Russia will agree to relatively favourable terms for Ukraine, Ukraine will lose Donbass but will keep the rest and may be able to join NATO and the EU. The counter offensive would have contributed towards that end and might have given even more favourable terms, it would not have ended the war, Russia would take everything back they lost even if that took years but it contributes to the effort that Russia has to expend.
I believe that if the effort needed becomes too great, Russia will eventually settle for peace once it deems it has achieved enough to sell as a victory and to achieve at least some of it's goals(such as taking the Donbass).

But taking Belgorod would never have given Ukraine a bargaining chip, Russia could never sell any peace brought from that as a victory no matter how hard it tried. Not only that but Ukraine taking Russia land(actual Russian land) would have a huge effect on the population of Russia, after all, as it is the war is still an external one.

It would have huge geopolitical ramifications for Russia as well as internal ones and Russia may feel that the only way to save face would be to take Kiev and achieve total vicotory.