Until a few days ago there was no intention publicly that Ukraine would ever move into Russian territory.
Even if Zelensky really believes the Russian military is that thin they'd have to retreat it does not give him the right to emulate what Russia is doing even if it's on a smaller scale. There are real people, Russian and Ukrainian, who live in these areas on the borders that are suffering immensely already.
You're viewing this in a 2D dimensional way, whereas war and politics and three-dimensional - it's all a big game of chess. This is a pawn in the greater game.
We're unsure of Ukraine's long-term objectives beyond regaining the sovereignty over their territory, and forcing a situation where they can maintain this.
How to achieve these goals comes down to a combination of military operations and diplomacy; to have a strong-hand in the latter, you need the former.
Purely speculation on my behalf here, but I view this offensive as having four main objectives, which overlay each in part - mutually supportive so to speak.
The offensive could be aiming for the key logistics routes nearby which, if captured, will have a negative impact on their abilities elsewhere on the front.
Is the offensive forcing Russia to redeploy assets from elsewhere on the front? Yes it is, and this could mean a negative impact on their abilities elsewhere.
Do you think the moral of the Russian army has been enhanced by this incursion? Likewise, do you think the moral of the Russian people has improved?
Like déjà vu perhaps, this could all have a negative impact on the Russian army's ability to conduct operations elsewhere, which likely means Ukraine territory.
It also strengthens the hand of the Ukraine diplomats to hopefully, at some point, be able to come to a peaceful negotiation, which is palatable for all.
While we cannot get carried away about this operation's success or ability to induce change, I do envisage an embolden Ukraine hand when it comes to diplomacy.
Russia is losing valuable weaponry, valuable infrastructure (potential gas pipeline?), and positions further into Russia like airfield and depots are now vulnerable.
At some point, we can now all hope that wise heads prevail and some people in Russia decide to come to the table with a fair and open perspective.
A purely defensive military and political strategy from the Ukrainians wouldn't produce this. If pushing further into Russia does, then it has my backing.