For me growing up as a Wii U owner. The point of no return truly came after Smash Bros came out and sales were still slow.
I remember during the Wii U era there was still a decent amount of confidence amongst Nintendo fans that the console would make a comeback in terms of sales once the new games came out like Mario 3D World, MK8 and Smash Bros. As people were thinking the Wii U will have the same fate as the 3DS where it started slow but still finishes strong once the games come out.
After MK8 came out and we saw Wii U sales jump twice as much that whole summer selling 60k units instead of the usual 30k before and even outselling the Xbox One in some instances, it gave me a bit of hope that sales would comeback once Smash Bros and the holiday season came around. However, after the Wii U still failed to sell that holiday, I think 2015 made it clear to us that the Wii U was done for, if Smash Bros and MK8 couldn't save it. Nothing would.
For the GC, I think the point of no return came after Windwaker was revealed to have a "kiddy" cartoony artstyle. OOT was a major success and system seller for N64 and the thing that could've appealed to the more mature audience on the GC which was Zelda turned into a kiddy looking game. Reinforcing the kiddy image that hurt the GC even more. It was clear that the GC was done for after that especially since the other major games like Mario Sunshine, Smash Bros, and MK released already up to that point.
However, you could also easily argue that the GC fate was sealed even well before it launched. Nintendo gave their biggest competitor Sony a huge 18 month head start for the generation. People already trusted PS way more to start the gen after being hugely successful with the PS1 and you essentially let them get an 18 month headstart. By the time the GC released, most gamers already made up their mind on getting the PS2. It was the sequel the hugely successful PS1 that contained backwards compatibility, the only fully trusted option when it came to 3rd party games, a DVD player which was the new hot thing at the time, and a much bigger library thanks to its head start. The GC's fate was set before launch arguably.







