OP has set a bunch of parameters that makes the discussion worthless, especially by excluding people who have different preferences. Basically, all arguments are invalid because they can be applied to other devices, and software aspects are subjective. What's the point of this thread?
If you prefer Nintendo games, the Switch is viable as a main, or sole platform, if you prefer other games, it's not, there's not much more to it.

For my part, all three consoles get very little playtime and I can't really see that any of them are anywhere near viable as the sole platform for me, or even main platform.

I couldn't agree more.

The average gamer just can't find games on Nintendo since the last years of Wii at least. So many genres, so many basic 3rd party games that Nintendo lacks, plus the prices and the technology. You can get a full entertainment machine from PS4/Xb1 and dozens of great games in a reasonable price, while many average gamers out there would only get Zelda on Switch. For most gamers out there, 2d platforms, indies, FE, Animal Crossing, Pokemon, octopath, don't mean anything.

I guess that's why many say "Nintendo doesn't have games". It's not as if it really doesn't, but it just lacks too much of the games that the average gamer looking for a console expects from a modern current generation console.

I thought that the question of this thread was exactly that: is Switch an equally optimal single gaming solution?

I agree, not equally so. All the third party games and franchises that one can think of you can find on both current generation consoles. Not on Switch, plus all other benefits that current gen consoles offer and switch doesn't.

Current gen consoles (ps4,x1) are like your regular grocery store. They're Walmart of sorts.

Switch is just that vegan, organic shop with a few of the items you can find on Walmart, but with its own quality stuff for its own public. Others can go there to buy posh 95% cacau chocolate once in a while, but when they need to feed, they go to Walmart.

How are they not AAA titles (I already said that Divinity is AA)? How do they noticeably differ from other AAA titles? They have similar graphics, sound, amount of content, etc. as other titles that are AAA.

But Octopath Traveler for example is not AAA. Or are you saying that it has similar production values as Just Cause 4? Or what Switch games do you mean?

Well my phone has crapped out and I lost my damn reply to half of the quotes.  So annoyed at that, and I have no time to type it all out again right now.

AAA according to people on this very site (some who are in this very thread) and definition is not graphics, content or quality it's purely a high mainstream and high budget which those games don't fall under, they're AA titles which is what I touched upon earlier and also what the person you responded to is highlighting. AAA releases are diminishing but it's not as much of an issue as AA titles are matching them blow for blow as far as the product goes.

If you're going to go by content, quality etc... then that means a whole host of Switch games as well are AAA.

I think the OP used the word viable at first but the context of the post was more in line with optimal.

As a vegetarian myself, your example is spot on. All of the veg/vegan options are for the most part viable for everyone in the sense that they could survive on them and enjoy good health. The vegan stores, eateries, etc. are definitely not for the masses though and that is all down to personal preference. So it is safe to say that something can be viable but not optimal, mostly based on personal preference. These same arguments could be made for cars, houses, or just about anything else where choice and preference are involved. Best overall is almost always subjective. Best for specific, well defined needs and preferences is much more fact based and can be supported by data and specific examples.

Divinity: Original Sin 2 is also going to Switch.  Also, Spider-Man, God of War, Shadow of the Colossus, and Detroit Become Human are PS4 exclusives.  That's like making a list of Switch games that PS4 and Xbox One aren't getting and including Legend of Zelda:  Breath of the Wild, Super Mario Odyssey, Fire Emblem Warriors, Xenoblade Chronicles 2, etc. on it.

How do you know how big budgets those games have? Just Cause 4 for example most likely has about as big budget as most other AAA games. It's atleast much closer to AAA than AA. And money that is spent on developing a game (not marketing) improves its quality and content.

Name those AA games that have been released this year that match these this year's AAA games quality-wise:

Monster Hunter World, Kingdom Come Deliverance, Shadow of the Colossus, God of War, Detroit Become Human, Vampyr, Spider-Man, Shadow of the Tomb Raider, Assassin's Creed Odyssey, Red Dead Redemption, Just Cause 4 (and these are only the ones that I have bought).

Those are all good/great games, so how are AAA games diminishing?

Well you can add at least Forza Horizon, Battlefield V and Call of Duty to that list (that's 12 games). And Xbox's lackluster exclusive games library is a well known fact and it gets a lot of flak from it. For majority PS4 is the primary console compared to Xbox. So why PS4 exclusives shouldn't count when we are talking about AAA games that are not on Switch?

Again some of the games you've listed are AA, SOTC for example would of had a budget comparable to games like Octopath which under the logic you're employing would make that AAA, Just Cause is a series that averages 2.5m per game so I highly doubt the development team received a budget comparable to games like RDR and GOW because that would mean huge losses sorry some of those aren't AAA games you're mistaking being good for being AAA, Just Cause is a mid tier title much like the Saints Row games.

Here are the top rated Switch games for 2018 as you can see the are over 50 games that have averaged 80 plus so the platform is hardly lacking in quality releases.

AAA are diminishing in not only release numbers but what set them apart lets look at 2017 where the were 15 actual AAA titles for the time 10 of which average 90+ and these were along side a tonne of AA titles, now days the are less releases from AAA and AA titles are going blow for blow with in everything but budget that's a sign of diminishing.