Google isn't like BNSF. If you need to move coal in Montana you're stuck with BNSF.
Google isn't like MidAmerican either. If you need electric service in Des Moines, Iowa you're stuck with MidAmerican.

The people choosing Google choose it because it does what they want.

People like to think they are rational and make decisions on such criteria. The truth is that there is, in the marketing world, something called “image momentum.” It means that once you have achieved a place in the consumers mind it is more difficult to dislodge it than if there was no image there at all. That also makes it harder for competitors to penetrate that mental “rack space.”

At the moment Google has the crown and probably will for a long time. Unless the company makes horrendous mistakes (“New Coke”) or is wrapped upside the head by some unforeseen circumstance (“Bud Light”), they will continue to coast on that momentum for quite some time.

People today forget that Schlitz was the #1 beer in the US in the 1950’s. Then a bright CEO in the 70’s found a way to make it cheaper and goose profits, except oops, it went flat sooner, so bars were selling stale beer. Profits increased! (For a while.) Still, Schlitz didn’t collapse overnight, it ran along for years more, outpacing corporate bribery scandals, stale beer, and “floating flakes” in the foam (a chemical attempt to prolong the life) and it has fallen to its current irrelevance. (Pabst, the most recent owner, finally discontinued sales of the brand just 3 months ago.)

Google could make quite a few mistakes and be OK, but not if it kept making them or made them worse. But any new contender would have to be better by a lot to dethrone them. (There are other business considerations which could hurt them; a dismantling of their ad monopoly [just validated but excused by the courts] or other unforeseen technical developments, but as it goes, Google will be around a while.)