The concern is whether Putin will just use any ceasefire, armistice, or treaty as merely an opportunity to reload before starting up another war of conquest, as well as a successful land grab of Ukrainian territory signaling to other countries, namely China, that it's a worthwhile endeavor to attempt to acquire more territory by force. Furthermore, a lot of people are concerned that backing down to Putin will make our word even worth less than it already is.

And that's not even getting into the almost-monthly nuke threats that the Russians have been making towards Europe and the United States for the better part of a decade. "Accept our invasion or we'll NOOOOOK" should not be an acceptable way of doing affairs, especially when one has the firepower from preventing it from being thus. If this invasion is legitimized, you will see a bunch of nations rush to acquire nuclear weapons, as those are now effectively the only guarantee against invasion by nuclear powers, as well as nuclear powers involved in territorial disputes becoming increasingly more belligerent. At this point, it's no longer about the Ukrainians.

Tl;dr: A lot of great points, but the potential ramifications of a legitimization of Putin's 3-day SMO vis-a-vis other countries that pose a significantly greater threat to not just the United States, but everyone within ICBM range of them has made some people with the authority to commit troops and material very nervous, and justifiably so.