[jcoby]

It's not really a psychological question. Boeing had no option to change the name and meet the requests of its customers.

The FAA says you have to have a B-737 rating to fly any 737, regardless of revision [1]. The MAX series is the maximum number of changes the FAA would permit and still allow it to be called a 737. In theory, someone who is type rated and familiar with the -100 series should be able to safely operate the MAX 8.

Most likely the airlines have rules above and beyond that (like requiring sim or right seat time in a particular revision), but this is the motivation behind the aircraft. If they had made a 738, it would have needed to go through certification as if it were a new design. By making it a revision, the certification process is significantly reduced in both time and costs.

1. https://registry.faa.gov/TypeRatings/ (note the MAX isn't present on this list; I assume due to it being grounded)

[darkpuma]

I presume that if Boeing arbitrarily decided that all new production 737NGs were to be renamed 738 with no engineering changes, on just the whim of some marketing team, the FAA would recognize it as the same airplane despite the different name. Perhaps they'd be quite annoyed with Boeing for doing that, but I can't imagine they'd actually require all pilots to retrain.

That's just my presumption though.