I fully understand the emotional and humanitarian satisfaction of doing that. But that doesn't make it a wise thing to do.
A NATO plane shoots down a Russian plane, that's an act of war, and gives Putin all the excuse he needs to attack NATO countries -- Estonia, Latvia, Lithuania and Poland. Under current NATO doctrine, an attack on one is viewed as an attack on all. Which means all of Western Europe and the US would be pulled in.
Good news is that he (Putin) can't handle the war he started, much less an expanded one. Trouble is, to survive in power -- and maybe literally to survive at all -- he can't afford anything less than something he can twist into calling a victory.
So since he can't handle what he has, and has to win in some form or fashion, the only way to respond to a shoot-down by NATO, and have the slightest sliver of hope for success, is with WMDs.
As much as I feel for the Ukrainian people, I don't think we're at the point of putting all Western Europe and maybe the US in jeopardy.
I think a more prudent alternative would be to provide the Ukrainians with air and missile defense systems -- probably should have already done that, but we can't turn back the clock now. If Putin does launch a WMD of any description, even into an unpopulated area just to demonstrate that he will use them, supply the Ukrainians with airplanes.
An A-10 Warthog would do things to a supply convoy or tank battalion that the word, "annihilate," doesn't begin to convey.