I'd be shocked if PS5 sells as low as 100m. Even with supply constraints it's outpacing the PS4, I see no reason at this point why it would sell less lifetime. I expect the PS + Xbox base will grow this gen to accommodate two systems both hotter than their predecessors.
At this point, I'd agree with you, but market changes can really impact sales. The N64 sold around 15.2 million units by March 1998, which was the first year to year and a half on the market (due to a skewed launch) making it Nintendo's most successful launch to that date, yet falling into a distant last place by the end of its generation. I wouldn't expect PS5 momentum to collapse in the way the N64 did, but I also wouldn't expect sales to be proportional to the PS4. Scarcity can also drive people to purchase a product earlier than they normally would - Nintendo's often accused of manufacturing scarcity to improve demand.
I describe myself as a little dose of toxic masculinity.