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.