Some people have speculated that as users move more to mobile devices that native apps will eventually replace all web apps, however, that doesn’t seem likely to me. Instead, web apps will likely advance to the point that they eliminate many if no most current native apps.
Apple really pushed the native mobile app market because it made them a middle man that got a cut of all app sales. This made Apple very rich while they had complete dominance in the mobile device That worked great for them but isn’t likely to continue into the future.
Generally, you need to have knowledge in a variety of languages for back-end development, the most universal is TSQL for database app development, and there are various languages frameworks that can be, and often are combined to develop back-end systems. Just pick something popular and keep up with any new languages and frameworks that emerge and are relevant to your field.