There's no way a cart full of goods will scan properly if you roll through a scanner. RFID scanners don't handle RFID tags being stacked very well.
Even decades old protocols support interacting with a plurality of tags simultaneously.
https://en.m.wikipedia.org/wiki/Singulation
(Edit: added wiki article discussing anti-collision, which specifically references grocery examples)
Just from these two technologies the "machine" can see that I picked one bag of crisps, one soda (oops wrong flavour I put it back and got the other flavour)(machine saw this and made the change).
RFID would only validate at the exit.
As for the cost 7-15 cents per RFID tag: -cash management costs. A lot. If you only do e-payments then you save from that -10 cashiers cost a lot (mistakes, skimming also costs)
Just from these two cost-cuts a super market can cover the cost to RFID everything in the store. Also the fact that a retailer (I am thinking Carrefour of Sainsbury's that move millions of items per day can get far better prices on tags).
I have read an article about race timing chips runners have in their badges during a marathon and they can apparently handle large number of runners, about 50 per second:
http://www.righto.com/2016/06/inside-tiny-rfid-chip-that-run...