With a few million copies sold on Steam alone (+MS Store +X) that is a very small number of players that stick to MP, and doubly so with the implication that rmode is the core feature that’s behind game’s success over the years.
And there are loads of additional factors I’m sure you’re aware of, like tons of people that bought the original version and its various physical re-releases, people that played HD, pirated version where stats are hard to grasp properly (piracy didn’t make MP impossible) and few others.
What you listed applies to most games that are being sold, regardless of genre, regardless even of having a MP mode at all. Most people don’t finish games, play a bit, fall off halfway etc.
Players viewing that situation through their own lens, and others in (relatively) very dedicated hardcore game circles, are having skewed view of the general market.
There’s nothing unusual about these participation Mp stats (especially in the context of one of the biggest, legendary RTS game of all time), and certainly they do not disapprove what I’ve stated before - the vast majority of people that touched AoE2 through the years, approached it primarily with singleplayer in mind, significantly less- with a plan to try to stick to MP/coop modes at all, and only a fraction- with a passion to participate in ranked games.
It’s like that in almost all balanced (content of modes) not only RTS games, but strategies in general.
Of course that greatly varies from game to game, and there are plenty of games with extremely short/uninteresting/boring/poor quality SP content (like single player campaigns in most CoDs), but AoE games were never focused or marketed primarily on MP, certainly not extreme end- ranked mode.
With the exceptions of course in the form of AoE:Online, but even there most of the content can be played alone to some point and it was (is- Celeste) a great time for players without friends, clans etc.
After that- probably III and IV are close together, because of their relatively plain and short campaigns. At least in 3(DE) there are a billion or maps to choose in the skirmish, loads of revolutions, minor civs, mercenaries, card system etc. that are a much better value. Not to mention civ number - even now updated IV is on the light side when it comes to content and felt stremlined/modernized from the beginning.
That’s obvious players of MP are the most visible and loud, but that doesn’t translate to the general preferences of people. I’ve spent a LOT of time on various modding forums, and from that perspective, circles of players interested in mod making, scenario creation and map design are not (well, have been when the game was fresh) that much smaller than a group of competitive MP fans.
It’s hard to hear or appreciate how many players just don’t care about MP at all. But they play alone or with friends, don’t argue on fan forums, don’t comment under Hera or Viper vids, don’t watch pro tournaments or any of that.
If it was even remotely true- it would be reflected in the contents Devs make. And SP campaigns, historical battles are as prominent as they’ve ever been. Even new maps can’t be pinned just on MP-focused crowd, because again- most people are casuals, and they might play said map a few times when they jump into skirmish mode.