Appeasement is not an option
Of course appeasement is not an option in such a conflict. However, the main problem is that there is no guarantee there will be any further U.S. response if Russia escalates militarily, possibly even before Javelin anti-tank missiles or other American weaponry were to reach Ukraine. That is why such a step may only increase the consequences and pain of war for Ukrainians.
A strong signal from the U.S. government is not enough if there is no determination for a subsequent military step. A completely different situation exists now than, say, during the Russian-Georgian war of August 2008. Russia is more motivated and the stakes are higher; it will not be deterred only by strong signals from Washington.
If the U.S. demonstrated a clear determination to escalate military support beyond a decision to provide arms to Ukraine, that would be another matter (leaving aside the risk of direct U.S.-Russian conflict and taking into account the nuclear factor). But in its absence? The Kremlin will have the advantage, even if just a propagandistic one, of escalating in Ukraine without facing real consequences from the West.
There is already no good option for assisting Ukraine through the provision of arms. At the current stage, any direct military-technical assistance will increase the escalation spiral without actually helping the country.