A mobile app is not a project, it is an ongoing obligation. The build is a one-time cost. The maintenance is a permanent line item. Most businesses that complain about "the ROI of our app did not work out" did not factor in the maintenance cost, because nobody told them about it during the sales cycle.
Here is what you are actually committing to.
Apple and Google release new versions of their operating systems every year. Each version breaks something in your app. Even if your app still works, the APIs it uses might get deprecated. You have to update to the new versions, or eventually the app stops passing review. This is annual work, minimum.
Device fragmentation. Your app needs to work on the new iPhone and the four-year-old Android that your customers still use. That testing matrix grows over time. New screen sizes. New notches. New foldables. Each one might break your layout.
Framework upgrades. If you built on React Native, Flutter, or any cross-platform framework, those frameworks release breaking changes regularly. Upgrading is work, and skipping upgrades eventually forces a bigger migration.
Store requirements. Apple and Google change their app store guidelines constantly. Privacy labels. Tracking disclosures. In-app purchase requirements. Every change is a compliance task your team has to do or the app gets rejected.
Security patches. Libraries have vulnerabilities. Your app uses hundreds of them. Every few months something critical drops and you have to update, rebuild, test, and release.
Customer support. Users report bugs. Users request features. Users one-star the app when something they do not understand happens. The support channel is permanent and nobody warns you that it needs to be staffed.
When we quote a mobile app, we also quote the yearly maintenance. It is usually 20 to 30 percent of the build cost, per year, ongoing. Many agencies leave this number out to make the initial quote look better. Then the client is surprised 12 months in when the app they built needs another chunk of budget just to keep working.
This is not a reason not to build a mobile app. It is a reason to budget honestly and to only build an app when the business case justifies the total cost of ownership, not just the build.