It’s the other way around. GrapheneOS users will be able to install all the apps but stock Android users will not be able to install apps from F-Droid and other alternative sources*. F-Droid will lose access to about 99% of the market.

*Technically it will still be possible for open source devs to distribute apps to stock Android users but it will require handing Google your personal information and setting up way more complicated build configuration.