[glimcat]

No, Gmail is a fermion. Therefore it is impossible for a second fermion with identical properties to occupy the same quantum state as per the Pauli exclusion principle.

Sorry, physics humor. Real answer:

Drive space is very cheap today. It is not hard to offer several GB per user. Email as it is currently implemented only supports messages of about 10MB each at max, while typical messages are text-only or include only a few low-res images as attachments. For typical users, it is therefore not probable that they will ever accumulate data at a rate faster than is currently the case since it is a protocol limit. Historical trends have also shown that larger file transfers are simply routed to other methods, so it is unlikely that email will be overhauled simply to increase the per-message size limit.

Monetizing it as a free service in a way which is not onerous to users and convincing users that they should use your service versus a more well-known and established provider are the hard parts. Solve that, and you've got a good competitor. Don't solve that, and you've got a money sink.