What? You've completely missed the point.
Everyone since the dawn of time has been able to go to websites like FPSBanana (which still loads slow because apparently it's 2004 or something) and download and install every custom skin/model/voice/particle effect they could possibly want. They can still do this. Thousands of people do. The problem is that such modifications are entirely client side, and thus can only ever be for personal satisfaction.
The point of microtransaction items like "cosmetics in TF2 and shitty mspaint reskins in CS:GO" is for other people to see your fancy hat/weapon. It's to show it off to people other than yourself. Some of it is conspicuous consumption to be sure, but a lot of people like customizing their digital avatars for everyone else to see. That appeal will never disappear. For good or for ill, microtransactions are the lifeblood of many games that millions of people play literally every day. The peak today for DOTA 2 was nearly a million players. League of Legends claims tens of millions of unique players monthly.
Of course valve shutting down these gambling sites is about money. It's also about gambling, but in reality gambling is about money anyways so... yeah. It's just not about mods like you erroneously seem to think it is.