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.
So I can disable a competitor’s email functionality by triggering a whole bunch of password reset requests for all discoverable usernames?
Together with cookies, you can show the captcha only to visitors that are not already recognized in some way, giving them a limited number of actions before showing the captcha. And regardless of whether you want one on your password reset page, you almost certainly want one on your login page anyway.