Going through a hard fork to exclude Bitmain and exclusive their own Obelisk miners is dumb.  First, it's probably illegal due to it being a monopoly and it is using anti-compete tactics.  It's kind of like if Microsoft changed their code so that Netscape wouldn't work as well in Windows vs Internet Explorer.  Oh, wait that was a real thing that happened and Microsoft was fined and restricted for years because of it. 

That said, nobody from the Government is going to bother to investigate this.  So, they can do it and get away with it.

But will it matter?  The guy with the lowest power cost will earn all the tokens, and the rest of the Obelisk owners will still operate at a loss. 

Further, Bitmain might just tweak their design to support whatever new algo that Sia comes up with.  The Sia team operate slowly.  By the time they announce a fork and what they are forking too, everyone will be adjusting their miners and tools to be ready for it.  If they want to bother.  The price of these tokens has been in the toilet lately anyway.

Maybe instead of concentrating their efforts on hardware miners, and forks, they work on making the application something that people actually want to use.