100% open rate on transactional emails feels too high to me. Something like an e-commerce purchase might kick off multiple emails (purchase made, shipped, arrived), none of which the user opens
Kicking off a chain of emails a user cannot easily opt out of could well be the sort of emails users want to lose. There probably should be a one-click 'stop emailing me' button, for this and future purchases. Which would be a support burden, yes.
We’ve received your order … we’ve taken payment for your order … your order has left our warehouse … your order has arrived in another warehouse … your order is with a delivery driver … all for a $5 cable.
So... let's assume many users do this, and let's assume Google factors in the opening rate into the transactional-email-likeness score, and that transactional-email-senders become widely aware of this...
Then senders' incentive will become to make the subject line into clickbait for the content, so that you'll open the message. So instead of subjects like "Order placed", "Order paid", "Order shipped", "Order out for delivery" you'll get uniform subjects along the lines of "IMPORTANT UPDATE TO YOUR ORDER". You will lose efficiency getting through your emails, and over time the metric will lose its indicativeness. Everybody loses.
Some of these emails are legally required for online shops. Doesn't matter if the user wants to receive them or not, they _have to_ be sent and actually delivered to the user's inbox.
I'm not sure how the 'actually delivered' would be enforced. Does Google have an affirmative requirement to deliver a 3rd parties message? I hope not.
My gmail address received 35 emails yesterday (which didn't get spam filtered). All but 3 of those got auto-archived by the filters I have in gmail. I would love google to just do this automatically.
Practically I might need another message or two a week that didn't hit my inbox.... but that's fine as long as it's as it is still searchable.
Sorry, to clarify, I only mean this particular type of transactional email: password reset, MFA.
But even for other types of transactional emails, like shipment confirmations, I would expect the open rate to be much higher and/or the complaint rate to be much lower than for marketing email.