The first step is to identify/clarify why they don't want to go further than 14? For example is it because of the Web client or because of C/SIDE.
If there is a wall that is preventing a client moving to 17, then I would rather have that client on 14 than on an older version. I feel this is better than them leaving the NAV world completely and moving to a competitor.
There are many NAV implementations that simply can not move to the Web Client (yet...)