Even if you can't renew a cert you paid for, in most cases you should be able to get a temp cert from Lets Encrypt and renew it every couple of months. I have free wildcard certs for many of my domains. HSTS just requires HTTPS. It doesn't pin the cert to a particular CA. That is what CAA records are for.
Are you saying that you have applications that require HTTP port 80 only?
It's true that letsencrypt makes this less likely to be an issue. But there is still the possibility that maybe your hosting provider drops support for HTTPS or you are forced to switch to a provider that doesn't support HTTPS. The parent gave one example of this with their GitHub Pages situation.
Also: HSTS applies to all ports once applied, not just 80/443. That is another important thing to consider before turning it on.
Are you saying that you have applications that require HTTP port 80 only?