Like others have said there are a number of Heroku alternatives out there that are more or less viable to meet your requirements, which I've gathered to be:
- Serving >100k users
- Helping you to achieve SOC 2
In my capacity as one of the leaders at a Heroku alternative PaaS, I've studied the PaaS market a bit to understand the space and available alternatives to Heroku. Here's what I've found about some of the most popular:
- Fly is managing its own infrastructure allowing it to be extremely competitive on cost. But on the flip side, its heavy focus on infrastructure is missing the “managed” options that make PaaS so valuable, such as a true Managed Database offering. This IMO makes it less of a viable alternative to Heroku.
- Render is offering a more truly “managed” alternative, and is innovating on cost as well. But it’s early and is still missing some table stakes reliability features that you'd probably expect from a Heroku alternative.
- Railway has a blockbuster FTUX, I love deploying and using PostgreSQL databases in the UI without even signing up. Coincidentally probably how it got to its count of 250k developers. But it’s own docs caution that it’s not exactly production ready, especially its databases.
- Platform.sh has grown really well by focusing on enterprise marketing teams and their use cases. I think this is a great niche for them and has paid off well. In their capacity of working with enterprises I'm sure they could handle SOC 2, etc.
- Aptible has run critical web apps and APIs dealing with sensitive data for hundreds of companies and has helped a few go public or get to billion dollar acquisitions. I am certainly biased, but Aptible seems to be the only non-Heroku PaaS focused on product/engineering teams that has repeatedly handled true "production" requirements, like your larger user base (many of our customers fit this description) and SOC 2 (most of our customers use our security & compliance dashboard for this). But that comes at a cost: Aptible is typically more expensive than the others, perhaps save for Platform.sh.
Disclaimer: I'm one of the leaders at Aptible, which in my (admittedly biased) view is the best positioned alternative to Heroku for product/engineering teams that have any sort of scale or production use case.