not so relevant for the shorter trips intra-EU, but still a factor
There would be rules around a fuel flight vs a legit passenger/cargo service, assuming they don't want to tax the extra fuel left in a plane at the end of a flight.
But that seem like a pretty straightforward problem to solve.
I'm Swiss and at least in Switzerland such a rule would automatically apply unless the population would do a referendum. And that would lead to a public vote that would almost certainty lose.
They aren’t going to russia for the Paris London flight.