Either NATO goes and fights Russia directly, which is highly unlikely

Yep, that's evidenced by statements going back to 2022 by senior US officials (including John Kirby himself at one point I believe) stating that even if Russia used a tactical nuke within Ukraine, we would not respond with direct military intervention.

https://www.nbcnews.com/politics/national-security/will-biden-putin-goes-nuclear-experts-say-nuclear-response-unlikely-no-rcna32756

If Russia used a nuclear weapon of any type, “I expect (the president) to say we’re in a new situation, and the U.S. will directly enter the war against Russia to stop this government that has not only broken so many international laws and violated human rights but also now violated the nuclear taboo,” said Evelyn Farkas, a former top Pentagon official for Russia and now executive director of the McCain Institute. “Putin will be signing the order on changing the regime.”

But two U.S. officials briefed on the issue did not agree, with one saying, “Unless they use them on NATO we’re probably not going to respond militarily.”

So if the use of nuclear weapons isn't a red line, then what is? And back to your point, what's the point of continuing this war if we're not going to intervene and it's only going to result in my Ukranian deaths? Personally, I think that the answer has been clear from the beginning. We're willing to use the Ukranian people as fodder to dwindle Russia's resources.