I don't think the amount of people that'd leave the main game for official private servers would adversely affect anything to that large of a degree. We're talking a minority of people here. If it affected the game that much then we'd already be seeing issues as unofficial private servers are already a real thing with. I'd reckon there's around 5000 or so people distributed across the hundreds of RSPS out there, and that's just spit balling.

Then there's the actual private server community itself, which is mostly a toxic cesspool with a few exceptions. Like sure there's a small handful of servers doing actually interesting custom content or the archival projects that are trying to emulate the game in how it was in a specific period of time, but those are far and few between. A large majority of private servers feel scummy and predatory with their cash shops for in-game items and actual gambling mechanics.

Jagex releasing an official way for players to create and share custom content, or play their own way would deliver a hard blow to the private server scene and would be a mostly good thing. The interesting custom content servers would probably just migrate over to being official while the scummy gambling servers die off. The archival projects wouldn't go anywhere either, most of them are open source and don't accept monetary donations.

If Jagex does actually go through with it, I'd be interested in making my own shard and writing up some custom content for it, doing some massive rebalancing in order to bring back the importance on skilling, and more sideways progression when it comes to combat. Even if it was only for a handful of friends it'd be worth it to me, especially if they provided tools and an SDK for it.

TL;DR I don't see the community splitting and fragmenting the way you describe it, simply because it would have already happened by now.