I never made that argument (strawman)
RCP8.5 is not business as usual. It has recently been snuck into climate papers and policy dicsussion as business as usual. It was never intended as that and in fact it's creators explicitly warned again doing so.
RCP8.5 assumptions are: #1 the switch to renewables will happen very slowly. #2 population growth will continue unabated. #3 income growth will be low, and #4 technological innovations will be slow to materialize.
The only one of those assumptions you could argue is likely, #4 (income growth could be slow.)
#1 switch to renewables is accelerating and as Bloomberg NEF reports point out is is already cheaper to install solar and storage than Fossils.
#2 Population growth is slowing or going negative in most developed countries. As China and India develop, their population growth will aslo slow. Every predicts this. See what I did there?
#4 Technology advances are accelerating and will continue to accelerate with AI and more tech getting into the hands of more of the global population.
So RCP 8.5 is most highly unlikely to ever happen, but most in the CAGW crowd do indeed present it as BAU.