Your post sounds like a softer version of the infamous "fall off a cliff" argument. If Switch doesn't accomplish something within five years, then it's never going to happen, or something like that. But new releases for Switch won't dry up and the existing library doesn't go away, so there's no urgency.
There's a Pachter version of "Switch won't sell 100m units lifetime" with the reasoning being that that would be 20m units per year which is absolutely not how console sales work. But this Pachter logic builds on the false premise that Nintendo consoles have short lifespans, so Nintendo must rush to get sales as fast as possible because there's a time limit. In that sense your post is comparable to Pachter's reasoning. With Switch being $300, it's a given that it will have multiple price cuts and such cuts are bound to happen once Switch is in danger of falling below 15m in annual shipments. The 3DS was an ill-conceived console and didn't have it in it to stay above 10m in annual shipments for more than three years, but Switch is a very different story. That's why Switch's current 13.5% advantage in LTD shipments will continue to grow by a couple of percent points each quarter, making 100m lifetime a pretty easy goal to reach.
The reason why Japan didn't recover for the 3DS like America and Europe did is that the 3DS was proportionally much more popular in Japan in the first place. The regional breakdown of the 3DS puts it roughly at 33/33/33 between Japan, Americas and Others, but the market sizes of Americas and Others are much bigger than Japan's. 3DS sales had a lot of room for growth in Americas and Others, but not so much in Japan where it was already on pace to become the third-best selling console of all time behind the GB/C and DS.
Eventually all console's sales fall off a cliff, and that typically happens when a new successor is announced.
My argument is a lot simpler than I have been stating so far.
1. All consoles have their own unique trajectory but...
2. Switch isn't outpacing fast selling consoles like the Wii (or 3DS in Japan)
3. Switch isn't outpacing leggier consoles like the PS2 or PS4.
So it seems apparent that you think it will outpace or match the PS4 in terms of legs, especially with a 125m prediction. You also keep talking about a price cut, but price cuts won't happen imo. If there is a price cut within a reasonable amount of time I would change my tune.