Ok, so let’s say both sides think it’s their territory, so neither will want to cause a nuclear disaster there. That seems like a good balance for everyone else on this planet, for the time being.

The difference is that Ukraine is now entirely reliant on the west to continue fighting. This creates incentives for Ukraine to continuously seek escalation that could potentially get NATO directly involved in the conflict. Incidentally, US senators already introduced a resolution calling for NATO involvement in case of such an incident

https://www.lgraham.senate.gov/public/index.cfm/2023/6/graham-blumenthal-introduce-resolution-to-address-threat-of-russian-tactical-nukes

Why is that?

Because that's obvious to anybody who's been paying any attention to what's been going on for the past two years. This ultimately comes down to manpower and logistics. Ukraine has far less people than Russia, and the west lacks the industrial base to continue supplying Ukraine at the current levels. This is well documented in western media by the way.

This was also understood long before the war started. Here's what Obama had to say back in 2016:

Obama declares Ukraine to be not a core American interest and that he is reluctant to intervene in the country, because Russia will always be able to maintain escalatory dominance there. “The fact is that Ukraine, which is a non-NATO country, is going to be vulnerable to military domination by Russia no matter what we do.”

The reality of the situation is that the west gave Ukraine everything they could for this offensive, and it will not be possible to replace that in the near future. The offensive was supposed to make a decisive breakthrough in the first 24 hours. In fact, this was critical for any sort of success https://foreignpolicy.com/2023/04/18/ukraine-russia-war-counteroffensive-attack-bakhmut-himars/

It's now been a month, and Ukraine has failed to reach even the first line of Russia defences while suffering horrific losses.

https://www.forbes.com/sites/davidaxe/2023/06/27/25-tanks-and-fighting-vehicles-gone-in-a-blink-the-ukrainian-defeat-near-mala-tokmachka-was-worst-than-we-thought/

The offensive is going so poorly that even CNN calls it disappointing

https://www.cnn.com/2023/06/22/politics/ukraine-counteroffensive-western-assessment/index.html

The most likely outcome here is that Ukraine will burn through their men and equipment, and once the offensive fizzles it's likely that Russia will go on the offensive of their own against a demoralized and depleted Ukrainian army.

John Mearsheimer has a pretty sober analysis of what can be expected to happen realistically that I highly encourage you to read

https://mearsheimer.substack.com/p/the-darkness-ahead-where-the-ukraine