That wasn't because of the concept, which clearly was popular, but the implementation.
Right, and now the implementation has been fixed to "normal ED release quality", how's the concept doing? The problem isn't that "space legs" aren't popular, it's that they weren't so universally popular that
everyone felt/feels they need the expansion, and there's virtually nothing in it for anyone else.
Horizons had some pretty terrible implementation misses too - the original Engineering caused a lot of ragequits, and a lot of future expense to fix up; Multicrew was a disaster that made the initial Odyssey release look functional - but it also had enough extras for a wide enough range of playstyles that most players did eventually buy it. If you're not interested in getting out of your seat ... what's Odyssey got? It's not nothing, but it's very definitely "wait for it to be on sale for £5" minor extras.
If U15 is released in a similiar state (which I very much doubt) it will bomb just as badly.
Yes, but even in that case, they won't be left with a £9M accounting writeoff as a result, which is a pretty crucial difference for when it comes to considering whether to abandon U16 or not.
Maybe I'm misunderstanding, but you seem to be suggesting that Frontier aren't good enough to cope with expansions like Odyssey.
Not exactly. More that the ED player base probably isn't (currently) large enough to make expansions like Odyssey economically viable, because the absolute fraction of active players who need to buy it is too high for the amount it would cost to develop - especially without the console players.
That could hypothetically solved by Frontier becoming 2-3x faster at developing ED features on the same headcount (though I doubt that's possible, and that's not a competence issue) ... or it could be solved by increasing the active player base by 2-3x first, so reducing the fraction needed to buy in for profitability ... or it could be solved by finding a combined DLC feature set which was a "must buy" for everyone, but which didn't require more than Odyssey-sized development, but that's probably not an intersection that exists.
If you want an "optimistic" view, I would guess that Frontier's current strategy for ED is roughly as follows:
1) Implement things like the Thargoid War - and maybe in the longer term something like Powerplay Mk 2 as well, given some of their oblique hints - which aren't too expensive to build and can be delivered incrementally, so that the game is in a good state to go mostly "fixes only" for a while (as they did, except for finally getting Fleet Carriers out, in the almost three years between the end of Beyond and the Odyssey release) but still mostly automatically generate some interesting weekly content for the more invested players and more variety for new players.
2) Hope that over a few years that approach increases player numbers on PC to a level which could make another big DLC potentially profitable. No guarantees of this bit actually working, but even if it doesn't, step 1 still strengthens the game and hopefully keeps the current levels of income/profit going for quite a bit longer.
3) If it does, start thinking
then about which ones of the many big features that they can't deliver incrementally it should be.
At best I wouldn't expect this to get the next DLC before 2029, but at worst I'd expect the game to still be baseline profitable in 2029.