Unfortunately, it's not just the battery life. IMAP IDLE is a relatively ancient extension to IMAP from 1997 — an era when nobody really envisioned it being used for mobile devices or even Wi-Fi networks, much less cellular ones. It's a pretty fragile connection that can be very unreliable on the various networks that mobile devices often find themselves on.I am at least happy to see Apple adding common sense features to iOS Mail that should have been done years ago. Now the question is when will they add IMAP IDLE as an option. I know people are going to say it uses more battery life, but given the size of today's batteries it should at least be an option that the user could turn on if they so choose.
I worked with a few email clients on Palm and Symbian devices that supported it in the pre-iPhone days, and it was generally a mess on mobile networks, and even some Wi-Fi networks. A well-behaved IMAP IDLE client would obey the recommendation in RFC 2177 and try to re-establish the connection every 20–30 minutes, but the connection was dropped so often on a mobile device that the net effect was that you might as well just set the mail client to a 15-minute fetch interval. While I'd expect that to get better with 5G, the dream of truly low-latency connectivity over 5G is still in its infancy.