The point is that the entire city is one big fortification, capable of hosting a garrison of immense strength.
If the Ukrainians post 50,000, mostly lightly armed, soldiers there, then surely Russia will need several times more troops to capture the city, with massive amounts of heavy equipment.
Defenders wouldn't just have the advantage of immense protection, but also the ability to move unhindered within the city, countering any assault and mustering local superiority in any area of the frontline. And the potential of outside attacks the siege would put immense requirements for securing the 90 km frontline needed for an encirclement.
We're not talking about Mariupol, which was lightly defended and easily cut off without much resistance. Nor are we talking about Gaza, which is similar in scope, but could be easily divided in sectors and with isolated defenders who has mostly basic light weapons and zero heavy weapons, plus no real outside support. Kharkiv is another type of beast entirely, with far more sturdy buildings than Gaza, a circular structure - preventing sectoring - well armed defenders, very difficult to cut off and with the potential for massive outside support.
The defensive position is such a great force multiplier, trying to surround it would quickly erode any Russian advantage across the front line. I'd think the only way of taking Kharkiv, unless the Russians want to raze all of it, is essentially to overwhelm Ukraine across the entire front, forcing them to shorten it.
