If you are spending less than $25k/mo on Heroku or equivalent, you're not ready to move off it yet.
A lot of times when I talk to people with a high Heroku bill looking to move to bare metal, I end up being able to optimize it by 1/3rd or more just by picking dyno types, scaling appropriately, and consolidating workers. You can't really do that when you're hiring headcount.
I'm curious, why not? Can't you do it on a regular basis, like refactoring?
What does my bill say about the performance of my app in an e.g. Amazon environment?
eventually if you're spending thousands per month on a PaaS it might be time to reconsider
B) Nothing in particular
It's not about the application or any technical concerns. It's purely a business decision - above $25k a month you have room to hire people and still save money, below that you really strictly do not.