The interesting question is what happens to the ~100k+ pods that never migrated to SPM. There's probably a lot of useful but abandoned code that smaller projects still depend on. This creates a bifurcation where legacy projects get stuck on older toolchains.
You'll still be able to use those - the CocoaPods repo will not go away any time soon. If someone wants to provide updates for those, it has to be migrated to SwiftPM. And in the meantime you may have to use both SwiftPM and CocoaPods, or fork & migrate those yourself.