If it worked this way Boeing would have responded to A320 back in 1987 with a clean sheet, but they did not.
They had just invested a significant amount in launching the 737 Classic and they also had the arrogance of an entrenched supplier.
I have a hard time seeing Boeing deciding to do two clean sheets, just after a new CEO came in and shut down the one clean sheet they were already working on.
Even if MAX's EIS had been flawless, the 737 frame has reached the end of it's evolution so Boeing would have had to replace it by the 2030s, anyway. NMA's intent was to make the 737's replacement EIS as smooth as possible because of the importance of the model to Boeing Commercial's bottom line.
The real pressure point for Boeing is 200-240 seats and while the MAX can hold the 200 seat line very well against the A320, it is clearly failing against the A321 above 200 seats and cannot reach as far as the A321 (230 seats vs. 240). So Boeing will have to clean-sheet whether they want to or not because 200-240 seats is where the growth is and I am not sure Boeing wants to risk launching the 737 RS "cold" and hope they do not have the production ramp issues they had with the 787.
Having a single fuselage design would allow for significant production cost savings at the expense of higher weight for the smallest variants, but if your smallest variant is 40m / 200 seats, I think it would be doable. A wider fuselage would allow for 3+3 seating in the smallest variants (200 and 220 seats) with a wide aisle to provide quick turnarounds for the short-haul missions these frames would operate on with 2+2+2 seating for more comfort and better servicing of passengers on the largest variants which would likely be flying the longer routes where turnaround times would be greater. And it could still offer a 3+3 higher-density seating option for short-haul hub-to-hub services with the quick turnarounds necessary for such operations.
The below-40m / 200 seat market would be the domain of it's own optimized family of 2+3 models which could go lower (perhaps down to the 70 seats of US carrier scope clauses).
I doubt we'll hear about anything new till after 777X has EIS which seems to be at best early 2021.
On this I am in agreement.