investors in airlines want fare growth but consumers don't. That is the reality in any business.
When your average fares are growing faster than any of your competitors, it will invite criticism from consumer groups and the government, regardless of the industry.
You have it the other way around regarding who is a threat to who. it is WN that is trying to keep from keeping DL from competing with WN, not the other way around. Given that WN has not grown its presence in any key DL hub, it isn't a surprise why WN wants to avoid confrontation with DL.... but let's be honest that WN has long avoided direct competition with the legacy carriers and pulled back from US at PHL as well.
The whole DAL discussion wouldn't be taking place if WN embraced competition at DAL by all carriers... but they don't want to do that, instead seeking a protected market where they can avoid competition, esp. from legacy carriers.