- Originally the conquest victory required eliminating every other victory-eligible faction. This took way too long even in vanilla, much less modded games, and it was always faster to do the diplomatic victory if you really want the endgame screen. So it became the majority-population and two-thirds heavy industry rules.
-- The intent of having the two different requirements was to create different strategies: snap a lot of enemy small planets, or punch out the core worlds with HI? Although I hadn't considered the "spam your own heavy industry" exploit.
- For the player I'd still expect a diplomatic victory to be preferable to conquest, with allies or otherwise, unless you're already associated with the biggest boys like the Hegemony and/or League. You can just throw a bunch of prisoners, AI cores and operatives at factions to get friendly with them (and kill off any small, recalcitrant factions). The main contribution of being in an alliance is to get dragged into random wars and otherwise incur increased risks of negative diplomacy events (the diplomacy 'disposition' calculation treats alliance members as having the size of the full alliance when calculating the dominance penalty, and this applies to intra-alliance relations too).
- Thinking about it a bit more, a problem with the diplomatic victory for NPC factions is that they don't try to win it at all. (They don't specifically try to win a conquest victory either, but tend towards it 'on their own' as territory gets taken during wars).
- All things considered, I'm vaguely thinking of (adding a setting to) remove all victory conditions completely, and let the player measure their own success. Don't a couple of Paradox games do this?
Hmm, isn't diplo victory kinda boring though? You just do missions, use operatives etc.
One thing that could also be (theoretically) done if you have a win condition that you have to maintain is to have some sort of "rally" when all non-allied factions try to prevent you from getting the win (e. g if you just spam heavy industry to rush for victory then you will probably not be able to defend it).
If you are thinking about removing all victory conditions... maybe then you can simply leave the conquest one? Most players might not go for it but it gives both groups (ones that want to "win" and others that don't care) an option of playing how they want (btw most people don't actually finish M&B because conquest takes very long too).
As for Paradox games: Stellaris has just a conquest condition (which includes the option of vassalizing others or adding to a federation). It actually had some others in the past but they removed them for some reason. Personally, Stellaris late game is pretty boring and tedious, I would prefer to have something other than conquest to go for, but oh well (they do have "crisis" events in the late game that keep you busy).
I think Crusader Kings doesn't have a victory condition per se, I am not 100% sure.
For Starsector it's probably better to have conditions that are not quite fast to achieve as the game is naturally pretty slow (all this rep grinding, exploration and colony development will always take quite some time).
A side note about alliances again: a player that's just commissioned by one of the winning factions should probably not be considered winning. Also maybe the win should just be for the alliance founder so that players are encouraged to develop their own alliances instead of just joining another.