It may have been a way to grind down Russian forces. The next few weeks will tell if this was also a trap for the Russians as the Ukrainians look to surround the city.
its too late for Ukraine to try to encircle, they would have to push through Russian defensive positions on both flanks
this is why Zelensky is talking down the value of the city, he knows too
You seem to be very knowledgable on the Ulrainian plans and overall situation. Either that, in which case shuttimg up would propably be prudent, or it is just forward applied 20/20 hindsight based on incomplete information and a lacl of understanding.