I think you are wrong. Not having a moat is a business model and a competitive advantage, and one which works well. As a customer, it gives me confidence that, if gitlab (or any similar company) screws up, I can host it myself. I go for open source precisely due to the lack of a moat.
That's what most open models rely on.
1) Hosted services are cheaper than running it myself, once I include staff time, so by default, I will go to the source vendor.
2) Even if there's a cheaper alternative, I'll gladly pay e.g. 50% more to go to the organization which wrote / maintains the product. Many companies just aren't that price-sensitive. If you pay $200k for a SWE, is saving $100/year worth it to go for gitlabknockoff.com instead of gitlab.com? Most big organizations wouldn't. The risk only comes in if gitlabknockoff was AWS, Azure, or GCP, which we're still learning what to do about.
3) There is a shallow moat in the forms of things like brand recognition, canonical URLs, etc.
If you're comfortable with e.g. a <100% profit margin, open models do just fine. Open models just mean you can't have a 10,000% markup or do an Oracle-style milking of customers. As a customer, that's why I pick open models.
Open models also mean I'm not SOL if you go out-of-business.
The friction comes in with a lot of hybrid models. Most things between open and proprietary don't work well. Datadog + gitlab are on opposite sides of this divide, and I don't see that working well.