It's tough to say, honestly, and it depends on how you define "gamers". The initial batch was all about gaming, usually centered mainly around a community (initially a single game (since there weren't many streamers around) but later also broader categories like speedrunning and competitive gaming). That actually held on for some years. But as the audience grew and grew, they opened the gate for alternative content like "IRL" which really created an opening for people wanting to serve (or exploit if you will) gamers and their needs (they used to have a policy in place where only actual gaming streams were allowed and you could report non-gaming ones). That had already been the case previously where camgirls/former pornstars streamed themselves "playing" a game, fishing for donations and the like. However, they never really fought that and in the end, just embraced it since it meant more money for them (via subs, later also bits as well as more traffic in general). That's not a wrong decision as a company, but it left enough people with a bitter taste in their mouth.

One of the main issues is that you basically have no filters. You can't exclude entire categories, you can't exclude individual streams. They have no proper tag system and they got rid of communities. All of that makes it more difficult to find new streams that might be interesting to you specifically. Then there is also the issue with performance. Twitch.tv isn't doing well in my browser and while I do have an "older" PC, other streaming websites don't have that issue. That and the fact that Twitch.tv is overloaded with pointless things I have no interst in ("new prime loot", despite having no such account, channel points which was mentioned elsewhere already, lots of notifications in the chatbox about who donated most, gifted most etc etc) basically forced me to watch Twitch.tv and chat with others (connecting to Twitch server via IRC client) outside of the website. It does work really well for channels you know you like but you will basically never discover new ones without going to the website.

Youtube is worse in general and I feel part of the issue is that they tried to bake the livestreams into Youtube proper. While it's not wrong to leverage existing platforms, it certainly wasn't good the way they did it. Also, due to the network effect as well as streamers being tied to one platform, competition isn't much of a thing anymore. As far as I can see it, the competition is long over and Twitch.tv won. The only real challengers were in the Justin.tv days when websites like Livestream and Ustream were actually larger and in the early Twitch era when own3d was still able to compete...until they ran out of money. As for livestreaming platforms for people who are into gaming only? It's possible that one such thing can become relevant even with Twitch.tv around but for that to happen, Twitch.tv would have to be enormous...and even then, they would still offer a larger potential audience. Still, if that happens, the emerging platform might have more of the "pure" gamers around who stream for fun, rather than for money (one of the larger issues I have nowadays - lots of people streaming to make money, having overlays practically begging for it).

It depends on what year you are talking about. I still fondly remember the times when Justin.tv was full of illegal cough restreams of the Simpsons, Futurama etc. I even paid for a "pro account" to get preferred treatment when it came to data transfers aka I paid, so that the streams wouldn't lag (and yes, that was a thing back then).