As much as we like to consider consoles as these closed exclusive tech kind of things, a lot of it was externally designed. With Genesis/Mega Drive, if a TMS9918 gets you most of the way there and it is better than what else is out at that time, then that is good enough.
When the goal is just to put pixels on screen at the pace of the scan line, a TI part will have solved most of the nagging base issues issues and it is easy to build on. No need for a clean room design.
Yes and that would be why Sega used it in the SG-1000. TI was no longer updating the VDP so later it was evolved in the Master System which was then evolved to the Genesis/Mega Drive. So to be clear the Genesis/Mega Drive did not have a TMS9918 inside just something that was similar enough to get a foothold into reverse engineering the actual VDP.
Absolutely, I didn't think it was the TMS9918 but it had a clear lineage from it. Like getting a modern Ryzen processor and realising that old 486 instructions are in there.
When the goal is just to put pixels on screen at the pace of the scan line, a TI part will have solved most of the nagging base issues issues and it is easy to build on. No need for a clean room design.