Its not that every e-commerce owner is moving towards mobile apps, People still make a lot with Ecommerce websites. A mobile app is for them who have a good budget and have a steady business, anyone without steady business from their ecommerce store should not think about making an app for their business. Their first goal should be to have stable sales.
Mobile app has its own benefits, when its installed on customers phone, you can send notifications via the app to customers phone free of cost, How cool would be to get free marketing?
If you are starting out, you should not even think this way as of now, start ecommerce store and get rolling first.