There's no possible way to justify Terran tech vs Protoss tech in any shape or form. The whole fighting amongst each other is largely detrimental as well as it means resources are constantly split. Besides, the Protoss did that at one point too (Aeon of Strife). Additionally, as Protoss have that whole unified thought Khala thing, development for new technology would be ten times as fast as Terrans could ever hope to achieve unified in the first place.
Battlecruisers are slow as fuck (using the cinematics as a reference), and apparently instead of laser cannons the Behemoth classes largely relied on small kinetic guns. I dunno what the deal with that is, but in four years the Dominion is not updating an entire fleet that fast. Not to mention that as soon as you start the game, the news says something along the lines of "Instead of upgrading the fleet you've squandered trillions on..."
However, at one point near the end, Horner says something like "Their shields haven't come up yet". I stopped and thought, "Wait, what? Since when do terran ships have shields?"
In NOWHERE are Terrans ever depicted as having shields. In the battle for Char, no shields are present. Mutalisks just rip their asses open like tin cans. So, if Terrans were able to fight Protoss in a real logical fight, we'd have to know what kind of shields that they possess.
Protoss shield tech can't be that good, or they'd have Planetary shields over planets like Auir. Incidentally, in my rewrite of Starcraft tech the Protoss do have planetary shields and this lets them hold off the Zerg for a significantly longer period. Terrans also have Battle Moons and space platforms. Now, they could still have stuff like that in normal SC fluff, but we have no idea. We have no idea how the Odin was even developed in the first place. Technically if the Terrans can mass produce something as big as the Odin only then would they start getting close to Protoss tech.
Shields alone give Protoss a huge edge. Also, their energy and psionic weapons probably don't care how thick your armor is or what it's comprised of. A fleet of interceptors could fly past a BC and hit the engines just like that. Wraiths or Vikings or whatnot have to get through a Carrier's shields. There's Yamato cannon, which apparently not all BC's have (dunno about sc2 timeline), but I don't have any numbers on what kind of energy a Carrier's shields could withstand. As all battles and arguments for this would start and probably end in space, given the Protoss' ability to just glass planets with a few Carriers, that's what it would drum down to.
What about Scouts? They fire anti-matter missiles. When I was writing a huge fleet battle for TOA I looked up the power behind Anti-matter.
The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×1017 J (180 petajoules) of energy (by the mass-energy equivalence formula E = mc²), or the rough equivalent of 43 megatons of TNT. For comparison, Tsar Bomba, the largest nuclear weapon ever detonated, reacted an estimated yield of 50 megatons, which required the use of hundreds of kilograms of fissile material (Uranium/Plutonium).
Oookay. So, in my world the Undead have these tiny fighters made out of elemental energy about 60 meters across. They are totally filled with Anti-matter, jump next to targets, and ram into them and explode. Now, the Protoss don't have
suicide fighters carrying ludicrous amounts of anti-matter, but they apparently have outdated fighters called Scouts that liberally toss these around like candy. I don't think Blizzard did much research into the potential for power that anti-matter has in the hands of a fighter. The scout would be throwing mega nukes at everything it looked at. You'd be looking at basically a Yamato blast for every missile if not more. Again, in my world this is small fry, but to Terrans? I can't see a BC surviving a single hit (Referring to sc1 cinematic of a BC firing Yamato at another one which obliterates it instantly). Now, if they do in fact somehow have shields, that may be different.
But that's really all I have to base the power of Protoss energy weapons on, save the Carriers glassing planets. Obviously it wasn't enough to stop the Zerg who aren't even armored.
I don't really care what some washed up novel writer who isn't familiar with the universe or even the absolute basics of sci fi says, the Terrans can't fight the Protoss so long as they rely on basic kinetic weaponry. Their power output simply doesn't compare. Now, they do well against the Zerg because, well, Zerg tech kind of sucks too. All they really have is numbers. A fuckton of numbers. They aren't like Tyranids who fire beetles that eat through the segments in your armor and then your flesh in seconds, or consume entire planets in mere days. But there's still a shitton of them.
In short SC has no sense of realism or scale at all. T.T
One thing I'd LOVE to know, though, is how a Hydralisk can destroy a battlecruiser in orbit.