The last argument might not be entirely right, but it shifts the focus from users' perspective to business needs. From users' perspective, the choice is essentially between these two: whether to have just an app, or app and a website.
However, from Flipkart's perspective the question changes significantly. What is the net benefit for Flipkart (and not just Myntra) of having only an app, versus having an app and a website over the long term?
The key to answering this question is to first stop seeing the mobile phone and web as merely two different channels, but as two different businesses. Today, it might be tempting to think of mobile phones as just another channel. Some years ago, Borders or Barnes and Noble probably thought about retail stores and the web just the same way. However, e-commerce turned out to be a whole new segment altogether.
What if mobile is not just another channel to make your sale, but something that will completely disrupt the market? If that is the case, you wouldn't burden the people who are working on the mobile phone app with managing the website too. You will not let anything take their focus away from the mobile phone. Flipkart did just that. Its biggest cost was not losing a few customers, but missing out an entire line of business.
To get a sense of disruptions possible in e-commerce, consider three examples. They are not from the future. They are real.