What airline wants to retrain all of their 737 pilots against an entirely new type rating, when a short 'changes' course will do.
That is something i do question though. For example the 777 and 787 share a common type rating. A difference course therefore should be shorter from one type to the other than a new typerating. However it is in fact longer than a CCQ (cross crew qualification) from an A320 to A330 whereafter you may fly both types in mixed fleet flying.
I guess since CCQ/MFF is an airbus thing that boeing doesn't want to do it and concentrates on a common type rating approach instead.
It is probably true that southwest and ryanair as the biggest 737 customers concentrate on least amount of additional training during the transition period from one type to another, however other customers are less than satisfied with this approach an are probably changing to airbus for commonality options between shorthaul and longhaul that are simply not available with boeing, despite a pretty common screen layout between the MAX and the 787.