The argument that people will always pay for the cheapest reasonable alternative would suggest that grocery stores would only sell the cheapest, generic brands. People will pay for quality and social proof, perceived or otherwise. Two identical products but one with (say) 500 genuinely earned reviews over time will always feel like a safer bet.
My expectation isn't that Amazon FBA will be my career for life. I expect it will be a viable income stream for a few years while I reinvest this time (and some of the earnings) into more stable industries (I have so much time to play with VR now) and develop an SaaS (including one in the Amazon space, go figure).