If you trade in a Kindle to Amazon, the value of that device may not be much, but it gives you a 25% discount on the Kindle you’re buying.
Paperwhites are occasionally on sale but the Kindle Oasis has yet to be marked down. So trading in an old Kindle is the only way to get a good price on the more expensive machine. Surely you know someone who doesn’t need/want their anymore or whose device doesn’t even work. Go for it!