I keep it just because every now and again my monzo card is not accepted. Coventry Building Society ATMs dont accept it (which is a shame as I live in Coventry) and I found a Pay-at-pump at Morrisons that didnt as well (which when your using it as it 24 hour and cant pay at the kiosk becomes an issue). I think its just a comfort thing - not sure what Monzo need to do to ensure its always accepted.
Also, I have Apple Pay on my old account - useful in the gym when i want a shake - dont have to take my card with me - can use my phone to purchase.