As SharePoint as a platform has been available since the early 2000s, there have been many ways for developers to work with it. As the SharePoint Framework's general availability and a more stable release were announced in early 2017, it is still very early for this development approach.
Currently, developers still heavily use the full-trust code, as this is probably what most developers are comfortable with if they've been working with SharePoint for years. Also, Microsoft has stated that the FTC approach is not being phased out and is still fully supported in SharePoint 2016. It might be safe to assume that certain elements of the FTC functionality will also be present in a possible future version of SharePoint on-premises builds. As such, we will see a long but narrow road for FTC developers, as most new and interesting capabilities will be announced as part of the SharePoint Framework. Developers working with older versions of SharePoint...