Here is the biggest open question: Can 2 custom game scenes exist?
Of course they can, and they are, even now. Sc2's custom scene is extremely small, vastly overshadowed by communities like Doom, Counter-strike, Arma, so on so forth. Custom content may not be what it once was in gaming, but it's still huge. The real question you should be asking is, "Why would anyone stick with sc2?" Other than community, I don't see a reason, between engines like UE4 for developers looking for big projects, and vast communities for other games more open to custom content than Blizzard has ever been. Sc2 is capable of decent projects given the time, but many other engines are far more open and far more reliable and easy to get into than sc2 can hope to be. At the end of the day, though, it all depends on what you as a developer want out of your project. If you're already situated with the game, then I see no reason to switch. But sc2 will not likely ever see its scene grow substantially after LotV's release.
The reaction from what I've seen is pretty extreme, that because the tools are not simple for either, it becomes a full time commitment to either and one can not participate in both at once. And this doesn't even go into the (IMO) irrational attachment or detachment to one company or the other (since companies are not people, don't know why people are loyal or hate them).
I wouldn't call it irrational. People get set in their ways. Additionally, many people are extremely hostile to Blizzard because of how Blizzard treated our communities in the past (their only communication was to issue C&D's to projects they saw as competition, they made fun of us for requesting them to fix wc3's problems during TFT beta, etc), and how they chose to reward our dedication with sc2's restrictions and censorship, not to speak of the charade that is sc2's editor when many custom content developers, including myself, went to extraordinary efforts to familiarize them with our polished user tools and why they were successful. It may not seem like a lot coming from wc3, but sc2 is a slap in the face to all custom content creators from sc1-d2 and anyone coming in from developer supported titles. Also, their extremely limited communication, things like "do you really want chat channels?", 1.5 silently deleting projects, all hint to a company that really just doesn't give a shit. I really wanted to like sc2. I really wanted to do something with it. I did my best to shed my opinions with past dealings and see sc2 with an open mind for five long years. But it fought me any step of the way, and in no objective, politically correct manner, can I see sc2 in any shred of positive light.
While I understand and agree with your point on the conceptual level - a company isn't a person so you can't really hate or be loyal to a name - what that company represents is an extremely negative connotation depending on where you come from and what you're looking for in a product. I look for objective improvements over old products, communication, and a standard of quality.
As a developer I see no reason to associate myself with anything Valve does, either. Nothing screams at me and says "pick me over the other choices". For someone already invested into dota 2 or the engine it is probably a very easy choice to make to move to it. For someone working on sc2, you should first ask yourself what exactly it is you want out of your project. If sc2 can't handle your project (like it can't handle any of mine), then maybe it's time to check out other games. If you're doing fine, there is probably little reason to switch, since it introduces a lot of barriers more likely than not to kill your project. Even an editor with perfect UI and responsiveness is nonetheless a titanic task to master. Dota 2's editor is just another fish in a very crowded pond. Nothing about it seems unique or appealing when compared to the other choices.
I've been at this for way too many years to have the patience for the kind of crap companies try to pull with modding and maps. This isn't 2001 anymore. There are endless examples to go by on how to foster a productive and positive community, and how to develop good and reliable tools in a reasonably short time, and most importantly, how to continue to support those tools and the community as time goes on. All of the countless years myself and others devoted to trying to communicate and establish ties to the company that made the games we developed content for a decade and more were an absolute, utter waste. To seal the pact, the friends I knew who moved into the company and then promptly vanished from everywhere else. That is how Blizzard does business. I just don't agree with it is all.
Ugh, this forum's bbcode is really the worst I have ever had to deal with. Why doesn't a double quote work?