Guy in another sub-reddit told me that the US has had such a high military budget for the last several decades because if there were a war in East Asia its probable that the US could lose a large proportion of its troops & hardware to enemy attacks well in transit the pacific. Is that at all true?

The question is severely restrained by the 20-year rule, but I can question his point here:

The large force levels are also the product of the realization that in most major conflicts since 1939 the attacker generally achieves overwhelming destruction of opposing forces in the opening operations.

Yeah, this thesis is valid, if you ignore the notable exceptions of the Second Sino-Japanese War, the Israeli wars, the Vietnam War, the Iran-Iraq War, and the Yugoslav Wars.

The whole reason why US naval doctrine requires such a large presence is due to the idea of air supremacy. As we saw in WWII, air supremacy in a naval theater can be achieved through either land-based airfields or flattops. The US has only a limited number of land-based airfields in the area, usually loaned by one country or another. In addition, some of the other airfields are not fit for the kinds of aircraft that would be deployed in a warzone. Thus, the US relies on aircraft carriers to achieve this air superiority. Carriers are vulnerable to long-range air attack as well as submarines, so a carrier in turn must be protected by ships capable of area defense (such as with the AEGIS system) as well as anti-submarine warfare (the use of escort ships and ASW aircraft). This results in one needing a large fleet to protect a carrier, forming the core battlegroups that we see today.

His point that the US is currently designed to be a "three-ocean navy" is valid, however.