I think this is the big question. It seems highly likely that Severodonetsk and Lisichansk are going to fall, and quite plausible that Kramatorsk-Slavyansk are next. Those are doable, though not guaranteed at this point. But then what? Unless the Ukrainian military starts to crumble from an organizational level, and this isn't impossible between the major losses of men and materiel, Russia will have many more uphill battles to fight.
The real "whittling" has come to the front line north of Kharkov. This is the first successful Ukrainian counteroffensive, beyond local battles, and it has operational implications for the Russian push. I suspect Russia will have to deploy fresh forces to the Kharkov front line. Some sources on the Russian side have already speculated that Ukraine had excellentintel, striking the lines shortly afterline Russian units were sent towards Izyum, and LNR reservist units were put in their place, in an attmept to "hold the line". It does appear that two LNR reservist regiments (or rather elements of) were involved in the fighting there though it's hard to be certain how much of a difference this has made.