I think it is rather easily predictable.
In order for MS to get back to the 360 days, two things have to happen. One, they don't shoot themselves in the foot. And two, Sony shoots themselves in their own. I don't really see either shooting themselves in the foot. Even with those things being met last gen, Sony was still able to recover and pass the 360 in the end. PS is just that large of a WW brand.
Sony seems to be making another traditional home console, while MS is throwing everything at the wall next gen (streaming box, low-powered and a high-powered boxes, both of which have a download-only box) hoping something will stick. The latter of which actually can lead to consumer confusion. If MS is advertising for 4 or 5 different consoles, but Sony has a nice all-in-one PS5, that also is B/C with their previous consoles at launch, the PS5 is going to sell on simplicity and features alone. Really, I think MS is going to continue to move their focus to PC gaming. They'll give XB2 a big push at the beginning of the gen, but if they meet a fate worse than the XBO, I think they are done with consoles.
As for the Switch, we already see its impact on the PS4 and vice versa. There isn't one. Sony is making a killing in the traditional home console market, while Nintendo is making a killing with their hybrid. They are different enough, and offer their own positives, that people are buying both. Sony isn't going to screw up their business model, that is extremely successful, to try and chase some other market. A market that has dwindled since last gen, and one that they just had a massive failure in. The most I could see Sony doing is some kind of cheap handheld streaming device that works with the PS5. It still wouldn't be a big focus of theirs. In other words, Sony is doing what it does best, powerful home consoles, and Nintendo is doing what it does best, unique handhelds (now, hybrids.)
Based on how predictable the Wii, 360, PS3, XB1, Pro, XB1X, and Switch all were, plus the more complicated the hardware, software, network, subscriptions, services, etc, situation get's overall in the console market, the harder it's going to be to predict in comparison to the past, which wasn't easy back then, as much sense as it may have made in hindsight.
Just giving an opinion on a portion of what may be causing the sales doubt. If none of what is rumored is true, or only the MS portion, then that doesn't change the fact that people may be concerned about PS4's continued sales longevity because of it.