Aside from the prospect of bad press and user protest - There are still non Play Protect certified Android phones being released, including a few rare phones that skip the Play Store altogether (including the Fire phones). So they could lose at least a little bit of ground in this area. In a sense, they are in competition with their open source offering, even though they have a lot of control over it these days.

It could also make jailbreaking more commonplace, which on the Android side has died down in recent years because sideloading is enough for most users.