The sites that do that won't be getting money from my and others' agents. Guessing that's going to become more and more of a problem for those sites.
And yours and other's agents will probably remain an insignificant and invisible customer base anyway.
My crystal ball is as good as anyone's, but if "agentic shopping" ever becomes mainstream, you can be sure that the vast majority will ask their phone (i.e. Google, i.e. Google Shopping) what the best price is anyways.
I expect a small site that undercuts the top Google results by 18% with a sign-up discount probably isn't profitable on those orders, so blocking agents would save them money - it's not like someone using agents in that way is going to have any loyalty to shopping from that site in the future.