Yes they do. They have to maintain their own branch, which may be very costly when new versions of linux would arrive. Successful upstreaming mitigates that.
Maintain? These are hardware vendors. Once it's shipped they rarely care.
They can already hide behind legal firewalls like the Chinese border. If the Software Conservancy continues, they'll just push more development to places where scofflaws can ignore them.
If they are OK with loosing western markets they sure can
This makes me wonder if they'd be better off pursuing complaints with Customs than with lawsuits.
Getting a shipment of infringing hardware impounded at the border would be a lot of leverage.