If the Max doesn't return to service or even if its grounding is prolonged this represents an existential threat to the likes of Southwest as part of the way they maintain their low margins is by having a fleet with one and only one type rating.
Other airlines (eg American) that bet on the Max (instead of, say, the A320neo) may suffer in the short to medium term too but at least they're better equipped to deal with this as they already deal with having a heterogeneous fleet.
Personally I never wanted to fly on the Max even before the crashes because (at least on American) they're even more cramped the the A321s and older 737s and 767s. I mean, who want that to shave off a few dollars? It's already uncomfortable enough. After the crashes? Why would I?
The leadership of Boeing needs to be ousted and, honestly, the DoJ should see if there was any criminal wrongdoing here as the response from Boeing (ie trying to blame the pilots and refusing to ground the plane) was abysmal. The fish rots from the head and if CEO pay is justified by the results they get (cough) then they're also responsible when it goes horribly wrong, which it did.
The big winner here is Airbus.
And yes, I fully expect Boeing to attempt to rebrand and salvage the Max. Personally, I hope it never flies again.
EDIT: removed EasyJet as they operate an Airbus not 737 fleet. Southwest and Ryanair however fly only 737 variants.