Paying a developer to implement a specific feature doesn’t do anything for the sustainability of an app, unless you think that because you paid, the developer is indebted to you for maintaining the feature.
Sustainability of an app depends on other factors, like creating a good developer infrastructure, keeping developers well informed on any upcoming changes that might be important, the ability to deal with bad reviews of an app, and most importantly, keeping the developer interested in your product (by offering sneak peeks, discounts, etc).
