I haven't seen any concrete plans about how they plan to distribute it when it's done, but I didn't get the same sense about them not wanting to upload it themselves. In fact, I think Google would be thrilled to push proper end-to-end support in Gmail.
End-to-end is a tradeoff. But presumably they wouldn't be working on the technology if they weren't prepared to offer it in some fashion. Probably as an opt-in? Hopefully a Googler can offer more insight as I can only speculate.
Sure they could, all incoming emails aren't magically going to become e2e encrypted.
If the goal is to make E2E more accessible then eventually more emails will come in encrypted. Maybe Google will let users with the extension find each other, or Gmail might help coordinate that.
Are people going to start filling in their pubkeys when they book a flight? What about when they're at the airport and need to pull up the record locator on a different device?
I'm sure more emails will start being encrypted, but that'll mostly be communications between people. Those probably aren't particularly interesting to google anyway.
The flight company (or email sender they use) could then request your public key and send off your message. And with some common protocols and APIs the complexity could be largely abstracted away.
I wouldn't underestimate the technology. When you look at more complex systems like internet routing tables, these problems feel a lot more solvable. We already have the technology for most of this. The problem right now is making it accessible.
If they think there is money to be made on this despite the hardness of the problem, they will come up with some okay-ish solution.
Or just make e2e 5 usd/month.
Though I never clicked on any ads in GMail, nor did I see anyone do it, so maybe GMail is not a great source of ad income.