Because exponential growth is unsustainable
Not inherently. It depends on the rate of growth and what is growing. For example, a 1% per year addition to bandwidth is exceedingly conservative, based on historical evidence.
it is bound to cap at some point in the near future.
Define 'near future'. Is that 5 years, 10 years, 40? And what makes you say that? It's easy to make a general unsupported statement. Don't be intellectually lazy. Show the basis for your reasoning, please.