The Ukrainians so far has not invaded Russia proper. There are a lot of ladders of escalation that the Russians can still do. Bombing the remainder of Ukrainian infrastructure; bombing Kiev; targeting higher up military leadership; targeting Zelensky and other Ukrainian political leadership. Hitting Russian airfields risk climbing such escalation ladder. The number of airfields are vast and repairs would not take too long. How much long range munition can Ukraine muster to fire at Russia?
Sorry what?
1. Russia couldn't even keep the lights out after trying that for a few months. It seems they don't enough munitions for sustained attacks.
2. Bombing Kiev militarily achieves absolutely nothing, on top of lack of spare munitions.
3. Putin will never target higher leadership, as it exposes him personally too and so it does for tons of oligrarchs and their families in Russia and outside.
4. Other political leadership is really replaceable.
Ukraine would have much higher priority targets than Russian airfields in Russia-proper.