why not linear growth, like +n MB per block halving, or quadratic like +n MB per n'th block halving?
Why would we choose linear growth when the trend is exponential growth?
Because exponential growth is unsustainable, it is bound to cap at some point in the near future.
We have no idea at what constant it will reach saturation. Instead we can try
a slow growth of the parameter, knowing that it will surpass any constant and thus probably
catch up with the real limit at some point, and hoping that the growth is slow enough to be
at most a minor nuisance after that.