We have dealt with this a couple times with shopping cart providers, mls idx search systems, do-it-yourself website systems, captcha systems, cms systems (paid-for, now with no upgrade path for things like PHP5 to 7), payment processing systems, and more I probably can't think of at the moment.
If it's not open source, and self-hosted, then we walk away.
The only things we kind-of lock into now are hosting providers, email providers, and domain registrars. But we use pretty big and established operations for these just so we hopefully don't have to move things.
Hosting... Don't do some crazy VPS thing, you don't want to worry about being a sysadmin. Dreamhost has a good WP setup with most things you'd need ready to go. There are lots of other "managed WordPress" companies too.
Email, just FastMail or G-Suite (Google Apps). Probably just the latter because it's familiar to most.
Registration is pretty negligible these days. I like Namecheap and their corporate values and support.
Does anyone know how they made their demo thingee on the front page? It's super slick.
I'm all for the headless CMS space, but I'll never recommend a solution to my clients that causes vendor lock-in and that I don't have the ability to operationalize myself.
Most folks in this space, wordpress included, follow a model of "open source, but pay us to scale and operationalize it for you".
They follow this model because it works.
As a CTO making decisions for my company, I'm not going to take the risk on a startup in this space without an open source fallback plan.