Yes, this “remote” backend approach would be the only remaining option. My belief in the possibility is bolstered by a recent collaboration with AWS to build their Service Management Connect, which effectively integrates JSM with multiple AWS services on the back-end. All without the need for Forge Remote.
That said, if you are not a company that routinely builds your own cloud services, like AWS, then the burden of not having the auth patterns provided by Forge Remote could make the back-end quite a burden. You’ll use Forge fetch to talk to your back-end, but you are still responsible for providing an appropriate auth model to make sure customer/user data remains secure.
In short, possible but non-trivial.