The US Army has been actively developing PA since the 1970s. If they feel that the investment is worthwhile enough that they have been actively working on it for forty years, it must be viable enough for their purposes.
I'm going to avoid the obvious F-35 comment and mention the
AIM-54 Phoenix.
Only fired in anger by Iran and only dubiously scored a single kill.
I could also mention the battlecruiser, the Yamato, attempts to replace the M16, or the Stryker. Forty years is not a very long time in military terms. It takes about a half-century for a new technology to be embraced or discarded and the rest of the century to be fully understood.
Powered armor is fundamentally flawed in that it takes the light infantryman, who's tactical strength is in concealment, infiltration and ability to use terrain to the greatest advantage, who's operational strength is in a minimal transportation, supply and maintenance demand, and who's strategic strength is being any sixteen to forty-five year old male five per-capita GDP invested per year. To replace this with a radar reflective, heat emitting, big, noisy metal box that requires about three or four times the transportation, fueling and armorer maintenance, and requires a much more highly trained operator...
Basically I don't see them as viable unless developed with weatherized stealth materials that bring the system to the same observability as a regular rifleman (who could also wear weatherized stealth materials) and a country reaches a per-capita GDP able to fund those systems with the productive labor of ten non-soldiers, which is itself a very generous allotment.
Per-Capita GDP for the US in 1991 was about 25k, a light infantryman costing 100k a year and an Abrams tank costing 212k a year. The rifleman himself only costs 25k a year (mostly pay) while the remaining 75k were in higher-echelon support, with ~7 thousand dollars equipment as a per-unit investment while the tank is a ~3 million dollar per-unit investment. Obviously specialists like anti-tank gunners use much more expensive weapons than riflemen.
It is plausible to have powered armor be economically viable if they have service lives comparable to tank fleets, but I really don't want to be the guy assigned a thirty-year old metal coffin recovered and refurbished after three operators were gooed inside. Furthermore the same per-unit investment could instead provide a traditional crewed vehicle; almost certainly more firepower, armor, endurance and speed for the dollar.
And all of this assumes a Western power with the hoplite view of the soldier; a soldier is a citizen, a citizen is of economic value, economic investment is made to preserve the citizen. A western tank crewman of a one-and-a-half child household with a $100,000 state funded college degree and hundreds of hours of training is the modern Greek cavalryman riding his household wealth into battle. When fighting an enemy that doesn't share this investment mentality (ie non-Westerners) there is nearly no end to how many militiamen with hand-me-down AK's can be supported for the same actual dollars.
So one might manage to produce a million-dollar powered armor with a service life that outlasts its operator's career, but its still taking the same $100 RPG grenade or an even cheaper 25mm cannon round to kill as an armored personnel carrier, without that vehicle's superior weapons platform, transportation capacity or speed.