I think the point being made originally was not that the leased planes will be returned (many articles suggest they won't), it was that these planes need ongoing maintenance and parts which the airlines won't have access to. The original tweet thread viraptor is referring to suggested that this is something that will hit the airlines much sooner than we laypersons might have imagined.
We'll see in the coming weeks, but the scenario you describe where a catastrophic decline in Russian flights such that there are enough non-Airbus/Boeing planes that can manage them is effectively a collapse in Russian civil aviation sector which is kinda what the thread was implying.
The thread in question https://threadreaderapp.com/thread/1498723248183382020.html
Even if all 500 planes are instantly returned at the end of March (how?), there will be reduced traffic to Europe due to the current European bans, and the rest of the traffic will be handled by the remaining fleet. I'm not even talking about Comac or other options.