Oh yeah, I definitely agree it's nothing new for them to be behind the curve. Again, I never owned any Nintendo handheld prior to the 3DS so I do't have strong opinions on those. All I can say for sure is that my 3DS always did feel like a second rate product, especially compared to the Switch.
Well the Switch is the first portable piece of Nintendo hardware to use contemporary, "premium" components. Had it been a pure handheld following that old design philosophy, we would probably see a K1 instead of an X1 and a Vita level screen instead of 720p, etc. The Switch is the first time Nintendo's portable hardware has been competitive with higher end products surrounding it. The traditional mindset was simplicity and affordability. But I do think the 3DS was at odds somewhat with the shifting handheld software design philosophies at Nintendo, moving from simple and bite sized to something more console-esque. It makes sense for, say, Kirby Triple Deluxe. It feels off with Monster Hunter and the like.