Not sure I understand your message. If you have a copyleft licence without a CLA, then the copyright is distributed between all the contributors, which makes it almost impossible to change the licence. And because it is copyleft, it means that the sources need to be distributed to the users.

So TooBigTech can build a service upon a copyleft project, but they have to distribute their changes, which means that the community can benefit from them. One example I learned about here is Grafana: AWS did not want to use their AGPL version so apparently they pay Grafana to get a commercial licence. That's of course possible only because Grafana own the copyright of the whole codebase. It wouldn't be possible with Linux, for instance, where nobody has the power to give a commercial licence and therefore it is GPLv2 for everybody.

It doesn't prevent TooBigTech from competing by serving the open source project, but that is more of an antitrust issue, I think.