I accept your point that good mobile data is not universal. But for those who have a personal mobile device and poor mobile data, an actual email client would

I accept your point that good mobile data is not universal. But for those who have a personal mobile device and poor mobile data, an actual email client would likely be a good choice. An actual email client uses much less data than webmail. For example the gmail webpage loads around 10MB just to show the web page, before any actual email content. Also an email client functions with an intermittent network, only actual sending/receiving requires a data connection. Existing emails are accessed using the local mailbox, and outgoing emails are queued until the network becomes available. A webmail system provides no functionality in the absence of network data.