Growing pains are never fun. It doesn't mention (at least that I read) if they're using HashiCorp Open Source or Enterprise. Open Source is great and I owe my career to it but they might be hitting the scale when the Enterprise features and support start to be worth the price.
I've only used Fly.io for a personal app but I think it's a great option so I hope they keep growing.
Agreed, any application regardless of language or framework can age poorly. I've seen some pretty solid "long running" Rails applications that were just maintained well.
Mostly Quiver (or Boostnote) with the files backed up to a remote git repo. Added benefit of being a format that is not locked in and can be parsed out later if required.