If the choice was "should we stop him now, or stop him later? Oh decisions, decisions", then of course everyone would agree it's better now.
But what about "should we stop him now by using means that could invoke WW3 and hundreds of millions, conceivably even billions dead, or should we apply maximum sanctions now and wait to see if we really do have a threat of further Russian expansion later, because actually we may very well not"?
A bit more nuanced decision I think. There's plenty on here who have the opinion that Putin is hell bent on restoration of the USSR. They key word is opinion. No-one actually knows, and moreover even if that was his aspiration, no-one knows whether he might have changed his mind after the shit storm he's led his country into already.
What if he genuinely is a deranged lunatic who is quite prepared to nuke half the planet rather than to lose in Ukraine? Is it a great strategy to just "hope" that he isn't? Fingers crossed, like?