A lot of RFPs are going out now that require a clear plan to update and keep apps current in a cost effective manner. This could range from strategic use of HTML/cross-platform tech, or some form of retainer agreement.

That being said, many app builds that I see being put to tender (at least in the UK) are often 'one shot' in combination with an advertising campaign or marketing effort. For better or for worse, many builds aren't designed to be sustainable or have a long-term presence.