Well the big benefit of the MAX is the engines, and the new engines have a much bigger fan, which means a complete rearrangement of how the engines are mounted on the wing, and how that affects the wingbox, which means in effect yes, they need the MAX. Boeing before the MAX faced the dilemma of the successful 737 form factor, but the requirement for larger fans for more fuel efficiency. They had to toss up between a newer replacement model (complete with more ground clearance, new design which is about a decade long timeframe, with the osbourne effects etc) or shoehorn bigger fan engines on the 737, which is what the MAX is.

In short if you want the ~15% improvement in fuel efficiency of modern engines with the 737 airframe you need a MAX. It is bordering on impossible for Boeing to take the 737 airframe any further after the MAX.