Looking at the direction this is going, I wouldn’t be surprised one bit that this is really the starting of WWIII. What if Russia really guns down a US supply plane? Nobody knows what Putin’s intentions are. The guy is 70 and may very well want to have his place in the history books.
Russia won't shoot down a NATO supply plane. For one thing, I doubt they are flying over Ukraine. If they shoot it down over Poland, etc., I believe NATO would immediately institute a no-fly zone over Ukraine. That would also include suppression of any AA assets.
Before all of this started people also claimed he would never invade Ukraine, yet that is exactly what he did. To me it is quite clear, Putin does what he thinks he has to do how irrational that may be. Hitler decided to go down fighting leaving Germany in total ruins, so why are people so sure that Putin is not in a similar state of mind?
The main problem with that course of action is that, once the ball gets rolling, there's a decent chance that there simply won't be any history books anymore for him to have a chapter dedicated to himself in.
A NATO supply plane being shot down over Polish/Romanian airspace would immediately trigger Article 5, pull all of NATO into the conflict, probably catapult the US into DEFCON 3 or 2.
This in turn the Russians would see as an "escalation" (completely omitting, as always, the part where they escalated first and NATO's acts are only a response) and then suddenly we might be hours away from staring down the abyss of nuclear escalation because Putin has fully gone of the rails and, while clearly being driven by irrational motives (c.f. his remarks on a "World without Russia not being a World worth existing"), probably still possesses a dangerous last bit of rationality which would most likely make him realize that, all own propaganda aside, the Russian Army in its current state would not hold a candle against NATO if the conflict were kept conventional.
NATO air forces would probably wipe out what remaining airworthy assets Russia has in the first 24-48 hours of such a confrontation. At that point, with your back against the wall and 70 years old, what do you do? Finally cave?
All that being said, I don't know what the right course of action is for the West.
Clearly, stopping arms shipments and leaving the Ukrainians out to fend for themselves while their Javelins run out and Russia (even if slowly, but eventually) manages to pick off the remaining Bayraktars should not be an option.
But the way a single irrational actor has seemingly destroyed the 60+ year-old game theoretic equilibrium behind MAD is scary.