I see your point, but I do feel in this case that this could be a feature that is a key selling point for Monzo. As I said before it is the one feature I miss from my legacy bank and keep the account open for (other than for writing cheques).
I don’t think it would be a big undertaking to partner with a small number of stores at first and with the growing market share of users it should appeal to many companies. Current cashback from other banks is very clunky and generally not very visible and I think this could easily be done in a much better way than competitors. This is achievable and can be profitable unlike a market leading interest rate, companies will pay for the exposure and Monzo gives back a portion of this to the user - and as I said with the rapidly growing user base I can see there being an appeal. The benefit for Monzo is another way to draw more users in.
Also I particularly thought it was worth bringing this request back up as a preview of this exact feature was posted on the forums by Monzo near the end of 2017 (Cashback). So I do think it is worth showing the interest so we can get an update on this and potentially get this back on the roadmap (which I assume it is not at the moment since it has been so long).