Dreamcast actually sold great its first year, it just was not profitable whatsoever and that coupled with Sega's already financial problems and the emergence of competitor systems for that generation killed it
Sega and Nintendo comparisons never make sense because even if Nintendo struggles with a console they have never been in debt like Sega was; quite the opposite- they are far in the positive in terms of cash on hand vs borrowed money; they don't need to borrow money.
also Sega never had the IP power that Nintendo does. even Nintendo arguably strongest IP of the 90s (Sonic) was pretty weak in popularity compared to a number of Nintendo franchises
at any rate the 3DS is selling well enough for it to not matter THAT much if the Wii U struggles
