If NATO declared a no fly zone, assuming the Russian government is rational and primarily concerned about about their military objectives, it would immediately

If NATO declared a no fly zone, assuming the Russian government is rational and primarily concerned about about their military objectives, it would immediately remove all their planes from Ukraine because it’s almost certain that the Western powers would obtain full aerial supremacy very fast. And any full scale attempt by Russia to contest it would lead to the destruction of their airforce. With ground forces it’s not that clear, Russia could still try to and potentially succeed to achieve some military objectives (especially if they are able to occupy any densely populated areas before NATO forces show up) because NATO countries would be 100x times less willing to sacrifice the lives of their soldiers and probably much less willing to risk civilian casualties. Also, assuming there is a guarantee (which is obviously impossible) I assume most people in western countries would support a no fly zone. I expect much fewer would support actual boots on the ground even given the same guarantee. Of course the risk of escalation is too great in either case..