I should have been more clear. It’s not impossible because the physical act of migrating. It’s impossible because so many other platforms are missing tablestakes features (HA Postgres with auto backups as an example).
React SPA front end. Hasura for GraphQL. Distributed Elixir (which Render handles brilliantly and Heroku is not even capable of). Also an old Rails app with web server & background job server.
I use RDS on AWS because I'm doing some weird stuff that Render hosted Postgres can't handle, but nor can Heroku Postgres.