They just have to make it hard/risky/expensive to keep using things they've declared deprecated, or to avoid the software they're pushing & steering. Which they've done pretty damn effectively (again, if this were all somehow accidentally, holy shit, it shouldn't have been, because, wow, it's been a highly-successful-and-effective set of business maneuvers executed over a span of years)

Doesn't matter if some hobbyists still run Gentoo with openrc and X-Window with Windowmaker and all the classic interchangeable daemons and components that Systemd replaced (and with which it is incompatible)—it matters that any commercial competitor's only realist option, short of a massive and risky investment, is offering the same things Red Hat does, but with less influence on those projects. They want it to be expensive to build a competitor that's not a Red Hat knockoff—and if that's not what they wanted, well, somehow they took a whole bunch of steps over a span of years to make it happen anyway, by accident I suppose, but the outcome's the same.