I am retired now having spent much of my civilian aviation career flying Boeing products mostly on the sublime 757. My licence shows that I was rated on the Boeing 737 100 through 900 series despite only having flown the 200 and that over thirty years ago. If I has spent my time with an operator flying just the 737' presumably I would have been able to convert to each 'new" version with the minimum of fuss which seems to have led to a scenario where the manufacturer recommends that differences training may be carried out by means of reading material on a tablet with no recourse to the simulator, CBT or classroom. Certainly I felt that towards the end of my time with my final employer, they felt absolutely compelled in fall in line with manufacturers methods of operating as opposed to decades of operational experience and this all down to the fear of corporate liability should the Operator choose a different procedure from the manufacturer.
Boeing were and indeed still are capable of producing first class aeroplanes. Their employees, customers and most important of all, passengers, deserve better than the level of management apparent at the factory.