Regarding the issue of Sonic and marketing:
The idea that marketing made Sonic is disingenuous bullshit, basically the video game equivelant of politicians blaming the media. Ads help sell things, but in the end people buy what they like. Sonic the Hedgehog was an AMAZING looking game for the time and extremely fluid. It flowed, as Bruce said, "like water".
When you put Sonic 1 (or the second one, Sonic 2 pack-ins sold more systems, probably) next to Super Mario Word it makes Mario look like a f*cking NES game. If Mario World is the begining of 16-bit and Yoshi's Island is the end, Sonic is the stepping stone that got us there. It raised the bar enormously.
If I was to take a cynical look at Sega's success with the Genesis I would mention EA and Mortal Kombat 1. EA and MK sold a lot of Genesis 2s, even in the later years when there was no benefit, and sometimes detriment, to the Sega versions. The train was in motion. Meanwhile, Japan, where they hated MK and didn't give much of a shit about EA sports, the 16-bit Sega years were as dark as the NEC years in the US.