An aggressor can prepare for war in few years and be ready for just a short period - this is much more efficient and affordable than to be ready all the time.

I prefer another approach to keep such aggressors at bay.
I'd prefer if we kept the know-how, developed equipment that can quickly be produced in great quantity, were alert with a moderate force and budget and ready to expand quickly.
Meanwhile, arms control treaties can keep costs down for everyone at conventional war crisis hot spots and alliance frontiers.
This is to some degree what some European countries do, albeit they fail at preparing seriously for the expansion phase.

The typical response to such a strategy proposal is the assertion that the world would run amok without the almighty U.S. military as policeman in the background.

Well, that's a very questionable assumption. We've seen most ground combat power of the U.S. committed to a war and its other ground forces being quite occupied with other than conventional war preparations.
I don't remember any country being invaded in the meantime (except Somalia by Ethiopia - which was obviously ENCOURAGED by the supposed policeman).

This suggests that almost the entire ground forces of the U.S. were not necessary to deter any aggressions at the very least during the past years.
Instead, they were used for the only major aggression in the past years.

I guess this should be debated somewhere else, as the French don't really seem to follow such a "prepared for everything" approach as their ground forces are not well-prepared for a major conventional war.

Has anybody statistics about the French 'defence' budget? Shares of personnel, equipment, operations and other costs?
Their insistence to develop many systems indigenously from usually just one company without competition might have contributed some inefficiencies.