I really don't see a reason to not use both. Even right now, a maintainer of some subsystem could conceivably create a repo on Gitlab* and accept PRs there, which they could then (rebase-?)merge in and sync with git.kernel.org with Linus never having to know about it.
The problem is cultural, as well as organizational. Firstly, I doubt any of the current maintainers would be willing to do that, especially without Linus's blessing and secondly, without a fully managed Gitlab* instance that takes care of sync and whatever else automatically (provided by the Foundation), this would be an unreasonable burden on the maintainers.