this " opportunities for expansion are finite" is somewhat true. the is an absolute upper bound on available land to build, and cash flow.
but in practice, they are growing and re-investing. the could push out fedex,walmart, bestbuy, other grocery stores, samsclub, costco, barns & noble, and many others. oh, and clothing stores, transportation sector like trucking, whoever makes corrugated boxes and bubble warp.
then they start buying out the producers, so no more proctor & gamble, just Amazon store brand.
the practicle amount of growth they could potentially sustain seems nearly unbounded, to the point of being almost infinite, within the constrains of time and space here on earth.