Unlike most of their competitors, I don't think I have ever heard of someone migrating to Heroku. Seemingly the only people who are still using it are using it because they started with it, usually with their free product. I'm no expert or anything, but it seems like a bad idea to cut off that funnel of future customers by eliminating that free product.
Otoh i know of a medium company (500 employees) that started in heroku and is moving to aws. Apparently they got to the top tier of the DB , and they've stumbled with several limitations by some lack of access .