In new product development, with a built in customer base with physical barriers to entry (interface parameters: ground equipment, maintenance, training etc), if you iterate a clean sheet design, you open up your customer base to direct competition.
If the airlines are going to have to invest in all new logistics and training anyway, now they can look at all the other competition out there. And that competition is going to be willing to make screaming deals to get the new business, and erode your market share.
I've seen this in almost every industry. Standard practice, a whole industry of business strategy consultants and their books exists to help companies analyze their market strengths, barriers to entry, and to maintain those barriers and create new ones wherever possible.