"The community" is often vastly overrated anyway IMHO. Patches are always good, of course, but people who actually consistently invest time in a project are exceedingly rare. You can't build a product based on sporadic patches. Besides, a lot of the time "the community" is just users (people with no contributions) complaining you're not doing stuff right.
Unless you have a viable (preferable proven) model that allows GitLab to hire developers and keep everything 100% open source, it seems to me this is the best and most balanced option. To the best of my knowledge, such a model doesn't really exist.
I continue to be surprised at the hesitancy (or even outright hostility) whenever someone tries to make an economically viable open source product, which usually involves things not being 100% open source because turns out, that's the only way to make things viable. "90% open source" is a hell of a lot better than 0% open source in my book.