Sure, all your arguments are valid. But in my opinion, there are still several unknown factors. Even if the new other consoles are not direct competitors, the "new thing" factor will definitely overshadow the Switch, and all the PR and articles will focus on the new gen. The image of the Switch could be damaged because it will still be considered "previous gen" (and kids, for example, might want "the new one" instead of the Switch now). And yes, it's a handheld, but let's not forget that the handheld market is shrinking more and more every year since the DS/PSP days, so we still have not idea what the size of this market is at the moment, and no idea about when it will be close to saturation. The Switch could do something awesome and make it grow again, but I wouldn't count on it too much, it would be a (good) surprise.
I'm not saying "the Switch will fail", far from it (I will surely get a Switch for Christmas anyway). Just saying that the Switch is in a new position compared to the usual models, and we don't know yet the limits and how it's going to evolve with new external factors. So I'm just curious to see what will happen. Could be great, could be less great, but in any case it will be interesting to observe.
Of Course, we dont know future, but everything looks very bright and positive for Switch, Switch is and its will be in very unique position on market despite PS5/XB2. Buy time PS5/XB2 arive on market Switch will most likely have price point of $150-250 with several revisions and very strong available lineup, while PS5/XB2 will be at minimum $400+ home consoles. Talking about shrinked handheld market, Switch is selling better than 3DS in same time period with difference that in same that period 3DS had huge price drop and one revision, 3DS after two years was at around 30m while after same time period Switch will be at 35m+. Also Switch like hybrid device can attract some of smartphone/tablet gamers because it could be seen like bridge from smartphone/tablet gamers to console gamers, like I wrote Switch has very unique position, not just compared to PS4/XB1 or PS5/XB2 but to 3DS also.
I already told you what will most likely will happen, mulitply revisions and price point of Switch platform, very strong Nintendo support with all Nintedo IPs on one platform, longer than usual Nintendo support, and plenty of 3rd party support (A-AA games, last gen games, Indies, and few big current gen games).