Is Mozilla an example for the next major adopter of Matrix or are they not? If they are, and everyone else signs up for modular.im as well: at what point does this ship of Theseus called Federation only exist inside a bottle?
Working with a third party lets them start standing up rooms immediately, and get their people using the new standard without being blocked on a long process of setting up their own infra. It’s federated. They can easily migrate onto their own hardware at a later date.
It also injects money into the community they are trying to support. Even if they did start running all of their own instances, I would hope they keep a support contract with Modular for that reason.
Paying Modular + using Matrix widely immediately + beginning work on their own infrastructure is a strictly bigger shot in the arm for Matrix than just putting a couple devs on pre-planning and prototyping infra would be.