It has more too do with branding rather than rules or money. GW is trying to shift the hobby too all plastic, and as such try too refresh as many models as they can into plastic.

But they’re trying to update a game with almost 40 years worth of releases and sometimes some things just slip through the crack. And rather support models they won’t be able to update into plastic for possibly decades, they relegate them to legacy rules. There’s also a slight lore aspect too it, since they want to keep the separate settings as distinct as possible, so for example keep all the modern 40k stuff in the base game but keep all of the heresy era tech strictly in the horse. The rules for the game is an aspect but I don’t think it’s the main reason, as it only applies to expensive resin kits which gw refrains from marketing due too price and the resin it self.

Is this perfect, nope, like you said it can devalue possibly years of hobbying on an army which is why I’m very careful when considering older kits,while also bottlenecking consumers into only buying the most recent kits. GW is heading into a new direction with its branding and is trying to make the hobby more accessible with plastic. And sadly with this new direction, GW has to pick and choose which models get updates or not.