Perhaps my previous post was vague, but I meant 'FAA requirements [of commonality, required to] maintain the current certificate'. The FAA may be in the right

Perhaps my previous post was vague, but I meant 'FAA requirements [of commonality, required to] maintain the current certificate'. The FAA may be in the right or in the wrong, but it has made certifying new designs almost prohibitively expensive and time-consuming; for evidence of this, simply look at the Cessna 172 (still in production on a 60-year old certificate), and what happened when Bombardier tried to put a new airliner into production. You're definitely right that the airlines wanted interchangeable type ratings for crew, but the issue slightly more complicated than you're painting it. I never argued the old certificate forced the issues, the certification system just strongly incentivized 'upgrading' the 737. This was one of many causes.