So according to that chart sales of both turbo chips and CD-ROM's were better as the system was around longer so why does it suddenly stop.
Huh? As the lines show, HuCard game sales decline significantly in 1992, because the format was fading out in favor of the SNES and PCE CD.
Also it doesnt seem like the Mega Drive and the PC Engine were that far apart....yes CD unit sales were a lot more for the PC Engine but still....if Sega had 15% of the market at its best than NEC might have had 18% and these are of course not actual numbers but still
Remember that all PCE Duos can play HuCard games, so for systems that can play the base games, the TG16 beat the Genesis in Japan by 1.4 million systems. Their overall market share for the generation is going to be a good bit above Sega's.
In terms of market share overall in the market year-to-year, though, the TG16 was #1 overall from '87 to '89, and was at its peak, as that chart shows, from '88 to '90. Because of its later release and slower sales, Sega was never #1; I think its peak of popularity in Japan was after the SNES's release, not before it like NEC's. (As for the PCE CD, it was the #1 selling addon in Japan that generation, so it won addons, but of course with only 2 million sold was no match for the SNES.)