Its a game. Ukraine can keep attacking weak spots, gain territory, use that for propaganda purposes, gain more Western and domestic support, increase morale in the Army etc.One will see how rational the Ukrainians actually are. They will definitely have a fairly accurate estimate of the Russians that they faced in and around Lyman, and they will know the rate at which they outnumbered them. As for the Russians, the Russians do not play hold on to any territory no matter what. The Russians could have withdrawn a week or two ago, but they probably decided to stick around to slowdown the Ukrainian general offensive elsewhere, allow for reinforcements elsewhere in northern Donbass and Kharkov to arrive and consolidate themselves for a future counterattack.
Hindsight is 20/20, and the Russians definitely did have the resources to have had at least 15,000 extra troops heavily equipped and on the defensive in Kharkov and northern Donbass before the Ukrainians arrived, that would probably have greatly limited Ukrainian gains. The Ukrainians have done well to do what they need to do and exploit Russian lack of man power and other weaknesses in that location.
This situation won't change until Russia brings in more troops in the front lines. That should happen be in 1.5/2 months