You bring up good points but I think we can predict Nintendo Switch sales with better accuracy than you think. Every home console (remember Nintendo themselves are saying this is a home console first and foremost) has sold worse than the previous generation home console. The only exception is the Wii. Its more likely the downward trend will continue than not.
This argument is silly, you're basically saying that there is a trend of Nintendo systems selling worse and worse every generation, but completely ignoring the circumstances of these generations. You're focusing on the conclusion, and ignoring the cause.
That isn't how things work. Every generation Nintendo has majorly fucked something up with either their console's design, courting 3rd parties, advertising etc which has led to gamers and 3rd parties going elsewhere.
For the N64 it was using cartridges over CDs, making it more expensive to put games on the platform, devs were also more restricted due to the cartridges limitations. This led to key 3rd parties like Square going over to PS One, leaving the N64 with mostly Nintendo titles.
For Gamecube Nintendo finally went with discs, but they decided to go with Mini DVDs, once again restricting devs because of the small size of the disks. The lunchbox design also came off as "kiddy" to many. It also lacked a DVD player like the PS2 which was a big deal at the time.
Wii Captured lightning in a bottle but sacrificed things like Power, storage, HD, building a good online experience and having an all inclusive experience like the PS360, thus it wasn't very attractive for the likes of Rockstar to put GTA on it.
With Wii U Nintendo was arrogant from the Wii and expected to get that Wii audience back
and a slice of the PS360 pie. Problem was they wanted the core gamers, but did no work to get them on their platform beyond releasing an HD system. They marketed it to a casual audience that had long left, and expected games like Arkham Knight to sell on it, when they did nothing to make the platform itself attractive to that crowd. This thing was bulky, ugly, and once again "kiddy" looking. Meanwhile, months later, Sony and Microsoft release systems where you can actually discern a jump in graphical power from the first glance, and (Sony at least) directly targeted the audience that would make their platforms successful.
With Switch, Nintendo are showing all the signs that they have learned from their massive failure with the Wii U. Everything about the Switch trailer show that Nintendo are finally listening after generations of being stubborn and out of touch.
The point I was trying to make was that its not a simple port like going from PS4 -> XBone. The architecture is different and there is a significant amount or work in making the game work when switching from x86 which is PC standard, and ARM, which is phone/tablet standard. Even if the power levels are the same there is still a lot of work to port. This combined with the fact that the GTA sells so good without being on Nintendo makes a really hard case for Rockstar to consider porting it. The extra dev costs will outweigh the potential sales and they won't do it.
Like I said, I'm not a tech guy, but people like Matt are clearly saying that this thing will have no problems getting ports from PS4/XB1/PC titles. If you're going to make a claim like this, please specifically tell me why an x86 title is hard to port over to an ARM based console. Like don't give me general stuff like "the architectures are different", please be specific.