Traditional generations are over people. Playstation 5 will come, but the majority of its games will also work on PS4 (with worse framerate and graphics). Microsoft will do the same, but because they can still lean on PC and xbox one x, they can afford to wait longer. They will never actually sell more consoles than Sony anyway, unless they shoot themselves in the foot a la PS3. And even then it was only 2 markets where 360 sold more, where PS3 did better rest of the world. Its all about maximizing profits, which is why Microsoft got in to PC again in the first place (biggest platform in the world by quite a huge margin).
The majority of PS5's games will not work on PS4, the Jaguar CPU is too big of a hindrance regardless of fps/graphical reductions. A large amount of cross-gen games will exist for the first couple of years or so as usual, just as they did with the 360/ps3 and xbo/ps4.
LTD: PS4 - 125m, Switch - 110m, XBO - 51m
2020: PS4 - 9m, Switch - 22.5m, XBO - 2.5m, PS5 - 4.5m, XBX - 2.8m