Design, engineer, certification, all cost in their own way … and in this instance, the cost of non commonality.
Although the flight deck displays appear spacious, its what goes on behind the scenes could be a problem, particularly the warning philosophy - integrated; new centralised warning display combining the critical alerts and logic (in this instance), or a distributed concept where alerts are still associated with the relevant system (existing aircraft ?).
See overhead panel.
The 737 Overhead Panel
Also not to forget that the original aircraft was analogue; how and where are the conversions to digital made or will have to change. What is the computing architecture, old new, processors, …
737 Centre Instrument Panel
This type of discussion would be ongoing throughout the aircraft’s development - when and where is there sufficient balance to change.
For the Max, previous balances might have been finely judged, but the 737 was ageing, Airbus had some surprises; and then there was the surprise of the extent of MCAS late in the testing. No dedicated MCAS alert, ‘failure’ had to be deduced, etc, … FAA will approve … etc …
Given this, it is more than likely that Boeing’s design was significantly influenced by operators; they wanted “faster, better, cheaper”, which aligned with Boeings requirement's, but then with hindsight how often is it concluded that this ideal is impossible, time and time again. We don't learn, or those that have, retire.