Gitlab as a company is about a precarious as Docker as a company. Most of its users are using it for free, and the few that are paying would happily pay anyone else who provided support for the same software. It’s a commodity.
But yes, many people also pay Gitlab for enterprise features (which are not open-source), hosting, support, and more. If they would pay anyone else then why aren't they using Github? If they just wanted git itself then why aren't they using the numerous lightweight alternatives?
Business isn't as simple as replicating a product, it's about selling value, and Gitlab has a unique solution that covers the entire software lifecycle. I'm sure you know all this but yet you've made numerous posts disparaging this company. Why, exactly?
It doesn't matter if you can clone it exactly tomorrow. Go ahead and do so. It won't give you the same valuation and it won't hurt theirs because that's not how it works.