I think the devil is in the details. Viability simply means that it can be successful as a console. Any of the current or previous systems released would fit that description in the sense that they can all play games. A better question would be is the Switch an equally optimal single gaming solution compared to the Xbox One, PS4, or a gaming PC. That is to say, is the Switch the best or most favorable single option.
In some cases yes the Switch is most viable but not equally so. I believe that if portability or a preference to Nintendo franchises are your primary preferences in gaming, then the Switch is the most optimal console for you. The Xbox One, PS4 and gaming PCs offer many advantages over the Switch that should factor in to which console is the best single optimal choice.
Xbox and PS4 obviously have specific first party titles that appeal to their fans and also have many more third party titles including almost all of the previously mentioned AAA titles. Many of these third party titles are not available on Switch and those that are available on all three consoles run better on X1 and PS4.
A gaming PC is optimal for those that want to run the available games at the highest resolution, frame rate, effects, etc. and also tends to be optimal for software price and availability of many older games. Of course a gaming PC will run AAA titles better than current consoles. Gaming PCs lack any of the first party titles from PS4 and Switch but have several of the high profile X1 titles.
So viability as a single console solution is heavily dependent on your gaming preferences. If you love Nintendo games, JPRGs, portability, then the Switch is the most logical choice if you can only choose one console.
If you love games like GTA 5, Red Dead Redemption 2, Fallout, Madden, Uncharted, Halo, Spider-man, God of War, Witcher 3, etc. or if you want to play games at higher resolution, frame rate, etc. then Switch is not logically the optimal system for you. It cannot play these games and it will always run third parties in lower resolution and lower frame rate when compared to PS4, X1 and a gaming PC.
"There are things which, if done by the few, we should refuse to imitate; yet when the majority have begun to do them, we follow along - just as if anything were more honourable because it is more frequent!"
-Seneca
