I think this chart does a good job explaining it actually. The fifth PlayStation year and sixth Xbox year had the highest sales.
I wouldn't be surprised if last year was the best year either the PS4 or XB1 had.
That's not necessarily a problem, mind. You can, as you pointed out, still ship a ton of units by having your best years up front and then slower years later.
The main implication is you have to replace your console platform notably sooner if you don't want the really problematic late gen sales.
Sure. I don't think this spells doom or anything. I just expect a transition by 2019 or 2020 at the latest because I don't think these systems actually have the sales momentum to keep going past that. Honestly, I expect things to start looking pretty ugly by calendar 2019 regardless, so hopefully it's not actually 2020.
Now, in Japan it might be more of an issue because the PS4's trajectory is more similar to the PS3 generation, so a "quick" flip is going to upset the market a bit, but I don't think that's a major concern when it's such a small part of Sony's business anyway.