THE:MILKMAN
Member
FWIW, the PS4 launched in late 2013 around a year and a half after AMD's Radeon Southern Islands line.
IMO, this is mostly down to lead times. Look at Xbox One X. The design had already been signed off before the E3 2016 announcement hence the specs not changing.
So if PS5 is 2019 it means the design will probably need to be finalised by ~ the end of this year/spring next year for a fall '19 launch. It just takes this amount of time (18-24 months) to get a product like this to production.