I don't know, I think retrogaming has been big for a long, long time. The first time I noticed was in the mid 1990s.  That's when MAME and other emulators first came along. I remember auctions hosted via newsgroups for vintage games for the Atari 2600, Atari 8-bit computers, Commodore 64, etc. before eBay was even around. I suppose retrogaming is becoming more mainstream all the time but it was a subculture (for lack of a better word) that started decades ago.