The upgrade question has for a couple of decades been one of the most complex issues with Navision, and I would guess it is the biggest issue plaguing Microsoft today.
There are many reasons to upgrade, from the customer perspective, some of the big ones are
So back to your question. In my 25 years of Navision I have found it extremely rare that a customer needed to upgrade purely based on new functionality. And I can not remember a scenario where the additional functionality would have any net gain for the company. The major reasons have seen for upgrading were performance issues, or compatibility with other software.
This predicates from companies having a single unified ERP system they they have heavily invested in, and the cost to maintain that is far less than the cost of an upgrade.
Of course this is all changing now. When eventually extensions are available as an actual solution we will see upgrading Navision as common as upgrading Excel. And the core issue here is that to get your system ready for that you will need to keep no more than one version behind.
In other words, there are many very good reasons to upgrade, but new functionality / features would be close to the bottom of the list.
Dear Greg, I agree with what you have written about extensions with NAV 2018. In one of our recent customer upgrade projects for an Investment fund company, we migrated them from NAV 2013 to NAV 2018 (with Extensions v2.0). Code migration was certainly not a piece of cake and we had to redevelop several customizations using Extensions. Nevertheless, the customer is now successfully using NAV 2018 with Extensions - and their future upgrade path is now significantly shortened!The top of the list customizations, where we had to redevelop include: