Indeed, Ukrainian resistance has collapsed to the point where Russia now taking multiple towns basically intact on daily basis. If Ukraine wasn’t able to defend well dug in positions they’ve been preparing for years, then there’s no hope of them holding territory where fortifications haven’t even been prepared.

It’s basically the whole concept with the Soviet deep battle theory. You push all across the front until you find a weak spot and achieve a breakthrough, then you push through it and attack behind enemy lines. At that point the whole line starts collapsing.