So it seems that come summer 2021 (Which I imagine will be a big one- more so than S20 was meant to be) they would be down by 10 aircraft.
You're assumption that summer 2021 will be bigger than what was planned for 2020 is a brave one. I would imagine that Jet2 are more concernec about getting through this summer than what they are going to do next.
However, if you are right I wouldn't have thought capacity would be an issue - the a/c being released by LH alone would be enough to cover your estimated shortfall. And who knows, even the MAX issue might have been resolved by then, releasing even more capacity into the market.
Long way to go before we get there, though. Far too many imponderables and uncertainities means it's all guesswork.