Only time will tell.
Companies do make mistakes but I'm still of the opinion that
you'd be a fool to believe NV will not pull this off. I'm hopeful the first RT game performs a little better than many are expecting and that DLSS will bring performance boosts too, once it's ACTUALLY implemented
.
Not sure why your brother has to justify his purchase? I picked up a 2080 instead of a 1080 Ti, hardly any justification needed IMO. Only around £100 extra currently and you get equal performance(slightly better) on average in current games, and future potential too. If the 20 series does end up being a failure and RT is **** for this gen, I won't be crying. £100 is a few nights out and still have a GPU that's a litter better than a 1080 Ti in current games.
I can see why some folks wanted a 1080 TI recently and why others preferred the 2080.