Centaur have been playable since Basic, they pre-date Complete Book of Humanoids. They weren't even the craziest thing in that book, Tall Tales of the Wee Folk had Pooka who had selective invisibility
If something has been playable for 35 years and, outside of D&D, is known in the fantasy ecospace as a common thing, it isn't a monster. Centaur aren't monsters or exotic creatures, they're just people, and this is a well running thing in fantasy. Like, it deserves being dunked on for all sorts of reasons, but when the absolute powerhouse that is Harry Potter has them around as just people, centaur are.... Just that. This is a massive, well running fantasy tradition. People will absolutely expect them to be playable, there's nothing about a centaur that makes someone go 'Yeah I see why that isn't playable'
5E's only introduction of new races are the MTG options, and two from Spelljammer, the Plasmoid and the Autognome. Everything else is a legacy race that was playable in 2E, and the two Spelljammer ones were introduced back then, just one was a mindless robot and the other had civilisations and stuff, just wasn't playable
I mean, I hate to crush your hopes and dreams, but the icebreaker HMS Baldurs Gate 3 just smashed right through 'em and cemented both types of Gith as a D&D mainstay for the next few decades.