What system was the biggest improvement over its predecessor (in overall quality, not graphics) and why?

[JackHandy]

Probably 3DS to Switch. Nintendo used slight-of-hand in convincing us that the Switch was a continuation of their dedicated home console business when in fact, it represented the discontinuation of it, and the beginning of their high-end handheld division and the consolidation of their software departments, which ushered in the perfect storm.