Oh and if you used AppEngine Standard, you can't move to Flex as Flex came with a whole new set of libraries. And Flex to CloudRun didn't exactly have a seamless migration.
Along the way they changed multiple internals with deprecation causing code base changes. Such-as Memecache, ImageAPI, Monitoring price changes.
I can't imagine any enterprise who's used the ecosystem continuing. The sheer number of architecture impacting changes leads to huge operational overhead costs to maintain applications running on this infrastructure.