They don't move to fast, they add "problems" slowly, because they know that if they do it too fast, people object heavily.
It's the whole "raise the temperature of the water to boiling slowly, and the frog won't jump out principle".
Before they can start with the kind of bullshit they pull, they need to get control of the huge majority of the market, until alternatives aren't really used anymore.
It's the tip of the iceberg, they're just starting out.
The next move might be something along the lines that, if you host a mod with them, then they'll add a "signature" to the mod, to make sure it comes from them. And you won't be allowed to host THAT version, with the signature, elsewhere.
That will of course ensure that pack made, and downloaded on Curse can't be gotten anywhere else, because it won't show up as compatible if you try to connect.
That in turn will prevent users who don't use Curse, to play with players who do use Curse.
Then they'll close the alternatives way to download the mods/packs, or those alternative ways won't contain the "signature".
This in turn will ensure that people use the Curse Voice Client, with the Modpack Add-on, if they want to play with the majority of the userbase.
At that point, the other launcher will die out, since pretty much 90% of the players will "have" to use Curse, if they want to play with anyone.
Then the ads bullshit will get even worse. The paid "premium" services will become more aggressive, the free version more shitty, yada, yada, yada.
Look, if you've been following the software industry for long enough (like, the last 30 years), these kinds of things are pretty much apparent.
Sure, there's some variation to the "schemes" some of those people pull, but it's always the same.
It always starts out with some form of great, awesome, suuuuper convenient help they provide to you, BUT it's always a closed ecosystem.
You can either use their thing, all of it, or not at all.
There no bullshit at first, so it seems great! It's so convenient.
What you don't realize is that you are trading your freedom for convenience.
Once you lose enough of that freedom, that when they start pushing other types of inconveniences.
But what are you gonna do? At that point, everyone is on this, everyone uses it, there's no alternative left, and getting an alternative to that point is almost a lost cause.
It's always the same thing, over and over, and it'll only stop when people finally start realizing that FREEDOM provides them the conveniences, not the other way around.
All I can do until then is stand back, and see waves after waves of people being butthurt when their "oh so convenient" thing stops being convenient, and say "sure, there's a little bit of upfront investment in the way I do things, but it's on my terms, so I'm always going to preserve my freedom."
What can you do anyways, nobody wants to remember history anymore.