It is not necessary to kill millions quickly. Killing them slowly would be more effective, a steady drip every day of several thousand dead regime personnel. Just keep hitting them over and over until it breaks.
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A nuclear response instantly makes conventional weapons pointless.
If nukes are going to be used, conventional weapons become irrelevant.
Use up the conventional stuff while it is still relevant and can yield a less insane outcome.
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Lastly, it is not a surety that Russia will be able to nuke Ukraine on its first attempt. NATO missile defense* can hit targets over Moscow with its SM-3** interceptors. If NATO gets lucky, they might be able to intercept a nuke before it hits Ukraine.
A conventional measured response is ideal to such a scenario.
*Aegis Ashore as deployed to Poland and Romania. Aegis as deployed to a whole bunch of NATO warships. There are a large number of interceptors floating around out there.
** SM-3's have a publish speed of Mach 13 and a published range of 1,200 km. This is a system designed to stop missiles in the boost, post-boost, midcourse, and terminal phases.