It is not just bandwidth or latency, and is not just for Antarctica. Not in all places of the world you have the best connectivity. Even with not so bad connectivity, you may have environmental interference, shared use or be just far from the wifi router. You may have a browser running in a not so powerful CPU, doing more things chewing processor, or the available memory, so heavy JS sites may suffer or not work at all there. You don't know what is in the other side, putting high requirements there may turn your solution unfit for a lot of situations.
Things should be improving (sometimes fast, sometimes slowly) in that direction, but still is not something guaranteed everywhere, or at least in every place that your application is intended or needed to run. And there may be even setbacks in that road.
I run into this daily on my phone. Where I live, its hilly, network is usually saturated, my speeds are crap usually and some sites more complicated than hn cannot even load at all without timing out sometimes.
Things should be improving (sometimes fast, sometimes slowly) in that direction, but still is not something guaranteed everywhere, or at least in every place that your application is intended or needed to run. And there may be even setbacks in that road.