If the argument is against fragmentation and against having to use stores with a less desirable feature set, then there really is no distinction between 1st and 3rd party games. Your willingness to excuse those titles is kinda arbitrary, especially for the developers like Activision/Ubisoft/EA that used to put their games on Steam and have now yanked them. Whether they are 1st party or 3rd party the net effect is the same for the consumer -- they end up using a client they don't want to play the games they do want. People want to argue on principle, but at the same time they carve out nonsensical exceptions to the rules where suddenly the principle doesn't matter because "Oh if you made or paid for the game then you can do whatever you want."
I don't know why people say something like "Blizzard made Diablo and is within their rights to distribute it how they see fit". TakeTwo/Gearbox made Borderlands 3 - why can't the distribute it how they see fit? If they made some janky-ass no-frills launcher and forced you to download it why would that be any better than having it on EGS?
The only reason that EA/Ubi/Activision abandoned Steam was because they wanted a bigger cut of sales and thought Valve's cut was eating into their profit too much. They had the infrastructure, talent, technology, and money to build a completely separate platform. But what about the hundreds of developers who don't have the money & talent to develop their own distribution platform? Why should they be obligated to use Steam? If somebody with a huge userbase is offering a much more attractive deal, why shouldn't they take it?
The fundamental difference is that games like Diablo, Fortnite, WoW, etc, were never going to be on Steam in the first place. Don't get me wrong, if a publisher decides to make their own games exclusive to their own platform, that's still shitty for customers, especially if that platform sucks. It's just not as shitty as paying devs/pubs to remove their games from other platforms. In one case, publishers are creating content to increase the value of their platform. In the other case, a platform is paying developers and publishers to reduce the value of other platforms. 2K would have never made Borderlands 3 an EGS (timed) exclusive had Epic not paid them to do so. As a consumer, that's not a good deal for me because Steam is better than EGS and games have more value on Steam. On Steam, I get achievements, cloud saves, user reviews, user forums, trading cards, badges, broadcasts, library sharing, universal controller support, Big Screen mode, Linux support, etc. On EGS, I get... cloud saves? Maybe? I think there are only two games on EGS that actually support cloud saves.
As for developers unable to create their own platforms, there's nothing stopping them from putting their games on Steam, GOG, Itch.io and EGS. In fact, that would be the ideal outcome for customers as it would let them enjoy games on their preferred platform. It would also encourage legitimate, pro-consumer competition. Unlike Epic, Valve has no issue with developers supporting other platforms.
The reasoning for a TakeTwo Launcher would be "TakeTwo wants more money from their sales." The reasoning for a TakeTwo game on EGS would be "TakeTwo wants more money from their sales." I don't see why there is some huge difference. The reasoning is pretty obvious and transparent either way.
Someone can't argue that they are trying to fight against fragmention and simultaneously claim that every developer who wants to avoid the Steam tax has to develop their own launcher. If there was a TakeTwo launcher, a Square-Enix launcher, a Microsoft launcher, a Bandai-Namco launcher, a TenCent launcher, and a launcher for every indie developer then the market would be infinitely more fractured and fragmented than it already is. People would still be forced to use feature-bare launchers. EGS ultimately prevents fragmentation by allowing devs to release their games on a platform with substantially better terms, without having to spend money developing their own platform.
Oh dear. Please don't use the term "Steam tax." It makes you sound like Tim Sweeney. Every distribution platform takes a cut. 30% is the standard cut used by Steam, GOG, Apple, Google, Nintendo, Sony and Microsoft. EGS takes a cut too. Epic monetizes Fortnite on most of those platforms so their talk of changing the industry standard rings a bit hollow. Speaking of cuts, I'm sure you're aware that the 12% cut isn't the reason why EGS exclusives exist. EGS exclusives exist
solely because Epic gives devs/pubs an immediate amount of cash for timed exclusivity. Let's not pretend otherwise.