^^^^^ Thinking aloud here:
If you are correct in this scenario, and Putin pursues the use of high-yield thermobaric (non-nuclear) missiles, the key issue will be his choice of target. If that is Kyiv, then Putin has deliberately targeted a densely populated area, murdering tens, if not hundreds of thousands of innocent civilians. Ukraine, the US, and NATO may see this as egregious as the use of nukes, and may decide to initiate a swift retaliatory response into Russia itself*, to take out its command and control centres, static missile silos etc', possibly even Russia's warships and submarines that possess the same launch capabilities. In other words - WW3.
On the other hand, if the response from 'The West' is to chuck out a few ambassadors from their respective embassies and have diplomats deliver words of condemnation, Putin will simply continue to pursue his policy of mass extermination of the Ukrainian population.
As has been acknowledged before - the only thing Putin respects is strength.
I sure wouldn't like to be in the shoes of Biden right now.
John
*edit: utilising conventional weaponry.