/vr/ - Retro Games

Time is the simpler and better metric. Everything else ends up too complicated to agree upon.
15-20 years seems very much acceptable. That is a measure of time used often enough to define something als old (in cars for example).
However, you have to look at the whole lifetime of a video game generation, not just the beginning. PS2 had way longer legs than most, so that is not very helpful. But you can spot the next generation more easily: the launch of the Xbox 360. I do believe the majority would agree there.
I'm fine with the /vr/ rules as they are now. I think it would be ok if PS2 would be considered retro 15 years after the beginning of the 360, i.e. the generation that followed PS2. That would be November 2020. Still enough time left.
Also Dreamcast is a bit of an exception, since it did not have a next generation sega console, being the last one.