From an operational and technology standpoint, I think the MAX is fine.
That said, I think the the MAX program just says a lot about the kind of company Boeing has become. Boeing has made my favorite commercial airliners ever and has an enviable amounts of talent, but sadly it's just not the company it once was. While the mission of all companies is to make money for their shareholders... Boeing seems to have made that it's sole focus, especially when you compare their aircraft to Airbus.
~ The 737 MAX-Profit sucks for passengers, it sucks for the flight attendants, and the cockpit is still just as cramped/uncomfortable for the pilots as it was decades ago. Add the fact that there's constantly long lines at the back of economy as 150 pax share two lavatories that one can barely turn around in. Like the longer NG's, it's been stretched and modified to cram, cram, cram -- and my understanding is that's part of the reason for the existence of MCAS. It's also why you'll see a pole propping their tail's up at the gate. It's also why they seem to need so much damn runway for takeoff/landing.
~ The 787 is great, but Boeing marketed economy seating as a comfortable '8-across' to the public, but designed it knowing all Airline would jam in a 9th seat they left just enough room for. The A350 was actually designed for 9-across, which is why it's cabin is 5" wider than the 787.
From the perspective of passenger/crew comfort, Airbus makes far superior aircraft. I understand that Wall Street and all the bean-counters at the Airlines love Boeing -- but I'm not an Airline executive, I'm just a dude that mid-status flier that loves commercial aviation.
Boeing indeed went from a manufacturer that focussed on the quality of flight, to a manufacturer that made flying uncomfortable, at least in economy class.
I don't agree with the many views that the Max will be the last iteration of the B737.
That's what everyone said about the NG, when they patched up the eyebrow windows of the Classics and stretched it to 189 seats, and offered the B739ER.
I think that Boeing could end up giving it another reengine and another stretch before a NSA is launched.
For the NSA, the engines are one potential point of development, but the B737MAX airframe is already very efficient as it is.
Even with active wings and new materials, it's difficult to make a tube with wings so much more efficient that it justifies a completely new design.
If they designed a completely new NSA today, with new lighter materials and all the possible aerodynamic improvements, the fuel efficiency gain from the airframe would only be between 3 and 10%. Would that small gain justify designing and certifying a brand new aircraft at a cost of dozens of billions?
As someone said, a lot will depend on how a step-up improvement in engine design will affect airframe requirements.
If they can install a 20% more efficient engine on a B737Ultra at the end of the 2020's, why bother launching a NSA for it?
A second question mark will be the market conditions.
A breakthrough in battery technology and global warming, high oil prices, "running out of oil" will become concerns of the past.
At that moment, aviation fuel will become so sustainably cheap that it won't make sense to pursue fuel efficiency gains and aircraft manufacturers will start to pursue putting that battery technology on their aircraft.
This is exactly what is happening with cars. We went from gas guzzlers to fuel efficient diesel, then gas-electric hybrids, then plug-in hybrids and now full-electric. No one is researching fuel efficient combustion engine anymore, the focus is clearly on full-electric now.
For aircraft, it will take a bigger leap in battery technology, but with time, it will come, no doubt about it.