The risk that your service becomes faang popular and you suddenly need unlimited everything and need it immediately?

It is possible but highly unlikely. The more likely scenerio is you just continue overpay like a lot of others waiting for the moment. If that moment happens you realize with the sudden popularity your store inventory is sold out so you couldn't profit off of the extra traffic anyhow.

There's a clear trade-off between downtime risk and cost explosion risk. For a hobby/non-profit project, risking the downtime to possibly save 7k€ plus surely saving the surcharge of "scalability" is definitely worth it.

The best setup will forever remain Heroku free instance tier with a free Pingdom account providing traffic to keep it from getting shutdown