1. The Wii U was an offputting piece of trash, that's why it failed. Software sells hardware, but that has its limits. If a console itself is perceived as good or at least neutral, then games sell consoles. But if a console is perceived as bad, things get difficult. A very obvious example of bad hardware is the Virtual Boy (atrocious hardware, actually) where nobody in their right mind would say that that console could have been turned into a success if only it had had more games. Wii U wasn't Virtual Boy level, but it sat comfortably in the bad zone. Mario Kart 8 (Wii U) and its Deluxe version (Switch) show how the same game can have a very different effect on hardware sales; Mario Kart's hardware selling power works tremendously better for Switch, because Switch is good hardware.
2. Those Youtubers say that because they don't like Xbox and would pick anything that suits their agenda. Would Xbox sell in Japan if it had more exclusives? Would Xbox sell as much as PS in mainland Europe if it had more exclusives? Of course not, because all those countries have a built-in bias against Microsoft due to MS embodying the deplorable attributes of the USA. How much exclusives really matter in the PS vs. Xbox console war can be better seen in the USA and the UK where the perception of Microsoft isn't the one of a villain. How much exclusives really matter can be seen in the bestseller lists of the PS4 and Xbox One. Exclusives do have a positive effect, but said effect is blown out of proportion to suit an agenda.
Switch is a different beast, because it sells without AAA third party games. Nintendo games are the main driver of hardware sales, and those games are exclusive by default. The conclusion here is that exclusives are incredibly important, unlike on PS and Xbox. Since both Sony and Microsoft rely first and foremost on third parties to sell their consoles, the debate about the importance of exclusives gets rather moot, because the biggest games of any given year are available on both consoles, barring the occasional exception.