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
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.
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.
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.