The logic behind not using them is either they can’t or they can’t. They didn’t kill a million russians and junk a fair chunk of their existing hardware for nothing. The reasonable perception is that they couldn’t take Ukraine and fight NATO at all whereas without NATO assistance historical or current Ukraine would have actually fallen in 3 days. Their ability to take Ukraine is therefore 100% a function of how well they can keep NATO out of it and nuking Ukraine blows that objective.