For me, Half-Life and Half-Life 2 modding was the golden age of FPS gaming. The life of a single game purchase was extended well beyond any expectations because of the creativity of the modders.
Unfortunately, mods like Counterstrike, Day of Defeat, and many others are getting developed and released as "Full games" now, to the detriment of the gamer and the industry.
Now we're in the age of relatively easy to use game engines, where anyone can develop and release a game, but there are so many games flooding the market that you look at and think "Why would you release this?". In the past, I truly believe these types of games would have been relegated to the modding scene and filtered properly through the communities to gain popularity naturally and organically rather than getting huge marketing budgets pushing us to buy the next big thing or FOMO.