Is there any universe where Putin just goes "Okay I failed" and just call it quit and doesn't use nuke?
I think the big positive in that situation is that Russia hasn't openly stated any of its objectives. The goal of the special military operation is to "de-Nazify" Ukraine, but that can really mean whatever anyone wants.
Like, Russian negotiators could go in and say "hey, we want out of this war and we'll give you a bunch of concessions, but could you make a public statement that Nazism is bad" and then declare that this is a victory and that Putin heroically forced the Ukrainians not to be Nazis any more.
Or, if you want to be a bit more serious, Ukraine could agree to expel some of the foreign far-right militants who went to fight there. Russian TV gets to run segments about how all these bad Western Nazis are being driven out of Ukraine. 100% success rate.
I mean, failing all else the government can just lie. Just make up some nebulous strategic goal and then claim the operation succeeded. The US has done that several times without even having the kind of media stranglehold the Russian government does.
The argument is always that if Russia use nuke they'll be blown up in response, but Imagine Hitler had some nuke, would he really have just blown off his head in a bunker rather that throw all the nuke around in a hail mary?
I mean, Hitler was very clearly a true believer. He absolutely did believe in the ideological principles his government espoused, and part of those principles was an intense valorization of self-sacrifice.
Putin is not a true believer. He's a corrupt, overgrown security service agent. The country he was born into is dead and he's spent the rest of his life feeding off the carcass. He's become one of the richest people on the planet by stealing from his own government. Moreover, Putin will survive losing this conflict. He has succeeded in eliminating all viable alternatives to his own rule and fostering a climate of political disengagement which severely limits any kind of effective organization against him. The balance of power within the nation might change a bit, but he's pretty much golden regardless.
There are people in the Russian government and military who are true believers and who would be extremely terrifying if they ended up in charge, like Igor Girkin. In that sense, Putin is useful because he serves as a check against the influence of those people.
Also, given that members of the strategic rocket forces have been encountered fighting on the frontline in Ukraine, it's also not clear what state the strategic deterrent is actually in. If the RVSN has been gutted for manpower, it's going to be kind of difficult to bring them to any kind of readiness quickly.