Apple email app is written for power users. Power users stay on top of their emails and rarely let it build to more than 20 unread at any given time. It makes this possible by having a very clean interface, easy way to delete or move emails and the ability to answer emails from just about anywhere.
When you have 2,534 unread, that's just being lazy. I have 9 email accounts on my iPhone 5S and 2 on my iPhone 6 and I never have more than 3 unread emails at anytime during my 18 hours of being awake with both phones.