The TF2 hats definitely proved the profitability of a cosmetic cash shop, but free-to-play or freemium games are older than that. Both RuneScape and MapleStory were early 00s and Turbine were also an early adopter of transitioning their MMOs from subscription based to freemium with both SWTOR and Dungeons and Dragons Online which at least initially massively increased their profit and sort of proved the viability of the model.
The TF2 hats definitely proved the profitability of a cosmetic cash shop, but free-to-play or freemium games are older than that. Both RuneScape and MapleStory were early 00s and Turbine were also an early adopter of transitioning their MMOs from subscription based to freemium with both SWTOR and Dungeons and Dragons Online which at least initially massively increased their profit and sort of proved the viability of the model.