Edit: I read their comment as FOR Russia not FOR Ukraine

>They will simply

Let me stop you there, not how Russian military tactics work. Nothing has been simple for them.

>They have very well been effective in their current numbers.

You don't pull out of a major city and fall back if you've been effective. You don't pull from Crimea unless you're desparate.

>Holding a defensive tactical position across a river is a fantastic position

Absolutely not correct for Russia. They have limited supplies, low armor, low morale, dwindling weapons, untrained soldiers and 0 morale. Ukraine pushed them all the way to the door of Crimea. The Russian Navy even retreated. Russia has steadily lost ground and gave up a huge strategic city. You cannot use the water lanes there anymore. It's a disaster.

>thinking from a defensive standpoint.

To continue, your defenders need SUPPLIES. They are almost entirely cut off from supplies with that waterway being a supply shipping lane that they've lost now. This is an unmitigated disaster.