It actually makes sense for Google and its users
Google does mine messages for ad sales, but it doesn't need ALL the messages for meaningful analysis.
Messages with political content, for instance, don't hold much value to google - it probably could be monetized at a stretch, but it's unlikely. Personal e-mails are more valuable, but you won't get/want most of your contacts to subscribe into the encryption scheme.
Much more valuable are order receipts, account statements - all the commercial fluff - that stuff won't be encrypted since the end-to-end encryption would need engagement from both sides.
So, if users would use this in conjunction with Gmail, Google would lose a small percentage data of rather dubious value and users (products on sale) would have an option to hide some of the important messages from the mining and other scrutiny, when it suits their privacy needs. Google would retain the user, who need the extra privacy. Everyone's happy.