FIRST RATE!

There's another permutation. Let's assume Russia has implemented a defense in depth. For that to work we must assume that ever line of defense fights equally when attacked. Otherwise the numbers don't work. As you point out, if you attack with 7 and the first line does its job than you have 5 remaining.

What if you don't try to get through but keep attacking to a stalemate. That would require Russia to keep sending troops to the front line (which I believe they're doing). At some point all lines must equalize as a matter of math.

The question is what is the ratio of losses for each side. Anyway, my point isn't that it's one situation or the other, only that you make a crucial point...it's complicated ;)

Also, I believe Ukraine consider the Bahkmut strategy a good one. Indeed, I believe it was so good they changed their counter-offensive plans. They're letting TIME do the work. As a chess analogy they're avoiding mistakes (loosing pieces) and picking off pawns one by one until they have a position from which to attack.