The quality of Sega's games (especially Sonic) has dramatically decreased after they stopped making consoles.
Ummm...not really?
Sonic declined for a while, definitely, but since 2008, he's been fine. The games have been of top quality and have had amazing production values. At this point, whether or not you like a Sonic game these days is down to preference of what you want from a Sonic game (do you like Wisps, do you like the Werehog, do you like switching between Classic and Modern Sonic, do you care about the stories and characters, etc), because the objective quality of the games is amazing right now. And he of course represents one half of the incredibly successful Mario & Sonic Olympics series, which is actually published by SEGA and developed primarily by their developers. And the Sonic and SEGA All-Stars games have been pretty successful, especially Racing Transformed, which actually sold quite a lot on Wii U alone (also it's popular on Steam).
And a lot of cult classic SEGA games debuted or became well known AFTER SEGA's jump to multi-platform. NiGHTS had a cool sequel on the Wii (I actually think it's quite a bit better than the original), Billy Hatcher debuted on the GameCube, Super Monkey Ball has been a pretty successful franchise with quite a few games in the franchise and also made it's console debut on the GameCube. Bayonetta is published by SEGA. Crazy Taxi has remained strong, and I hear Jet Set Radio Future is a lot better than the first game on the Dreamcast. And of course, SEGA has acquired a bunch of studios and IPs lately, like the Total War series, which is really popular. Also SEGA pretty much dominate the arcade business and pachinko business in Japan.
I could go on but you get the gist, SEGA products are of excellent quality and it shows.
However, I do want Nintendo and Smash Bros to remain exclusive for reasons listed by other users. I could see some potential benefits to Nintendo going multiplatform, but the negatives far outweigh the positives. SEGA were just a special case.