I’m not a “pacifist” in any meaningful sense - I despise war, but accept that it must occasionally be waged, and the United States must retain the capacity to win it. Nor am I an isolationist - I accept that America’s interests are global, and so our ability to project military power must be more-or-less global in scope. Thus, I would never seriously propose eliminating the carrier fleet (for example).
But the fact is that we just don’t fight against technologically advanced states any more. The Europeans are our allies, and we have no foreseeable basis for conflict with them. China is our trading partner, scrupulously non-revanchist (except for Taiwan), and nuclear-armed. Russia’s the only nation on Earth that could pose an existential threat to the United States, thanks to her remaining nuclear stockpile - but we pose precisely the same threat to them, and so war seems extremely unlikely. (After all, we never fought the Soviets, and they had a compelling ideological motive for war that the Russian Federation lacks).
So, leaving out our allies and states whose nuclear arsenals make war impractical, the only folks left are really marginal military players - places that face a huge technological gap, and (most important) don’t really have much of a prospect of closing that gap. Iran, for example, is unlikely to develop a fighter that can go toe-to-toe even with our older planes in service, let alone field an air force of such beasts. And Iran is probably at the high end of technical sophistication for the countries we might realistically wage war on.
So, given that we have this unassailable technical edge over anyone we might actually fight with - to what extent do we still need to invest resources in developing new weapons systems? To put it another way, which of our weapons are “good enough” that we can basically say we’re “done” developing them for the foreseeable future?
I’d suggest that fighter planes would probably top the list. We very rarely dogfight, and it’s fantastically unlikely we’d dogfight with anyone who has (a) planes on parity with ours as well as (b) a comparable training regimen. (I believe Pakistan has some fairly modern planes, but faces the same problem as most poorer states; training pilots is expensive work.)
Tanks are probably another good example - if you’re not NATO, Israel, China or Russia, you’re probably fielding ex-Soviet garbage.
On the other hand, I think there are some technologies that absolutely warrant further development. Foremost among them would be those relating to logistics - anything that make it easier to ship men and material quickly to hot spots is well worth doing. Cargo planes, ships, and so on. And of course, drone technology is hot - the ability to make things go “boom” with precision and without risk to American lives will continue to be valuable.
Big picture - with the exception of drones and perhaps one or two other weapons systems, I don’t see a lot of need to make the “sharp end” of the American military any sharper. We’re already as good as we need to be at making rubble bounce. On the other hand, the non-sexy logistical and cargo-hauling stuff will almost always be worth improving, especially since we continue to have global interests - and the better we are at moving resources around, the fewer of those resources we need. (You don’t need big garrisons everywhere if you can send troops wherever on an as-needed basis).