Just thinking about this here. Originally you had the Wraith, a ship capable of flying in space and engaging in effective ATA, ATG while in the atmosphere with the ability to cloak as well. It was retired for cloaked, only ATG helicopters(Banshees), and a flying Goliath which can only attack ATA or ATG, with the banshee equaling a Wraith in cost, and a viking being only 25 less gas. It just seems that the Wraith combines the best of both worlds, and that terrans seem to have actually devolved rather than progressed.
The Wraith's ATG sucked though. The Viking was more a goliath replacement (in lore) than a Wraith replacement, as far as I can tell. In lore, Vikings seem like they're ground units that can transform into air units, rather than the other way (which they are in game). I have to say, as AtA units, I think Wraiths were cooler, being (seemingly, in lore terms) faster and having greater fuel range. In other words, Wraiths are proper space fighters, and Vikings are not. However, Banshees are just so much more awesome than Wraiths. Being able to actually dish out damage against ground forces is important. In lore terms, you'd expect Banshees to do splash damage, giving them additional ground-pounding capability.
You could also take the approach that in SC2 every is simple more powerful than their SCBW counterparts. For example, the Wraith was decent with it's 20 damage missile attacks and 100 health. But in comparison to the Viking, who would do (simply for purposes of lore) 50 damage with it's missiles, have superior armour and 300 health; the Wraith simply couldn't compare. I suppose you could apply this to returning unit's aswell, like how the marines have different armour now (whether that was intentional or not) and thus have 5 more health. For the Protoss, they may have bettered their personal shield technology, and now that so many Zealots have died, there are only the best now. Thus, their are more resilient, and do more damage. I guess this is kinda confusing, and really would be hard to justify in the campaign-where you have new units right next to the old ones. But I'm sure if you were better at explaining ideas than me it would all make sense
It's easy to justify in the campaign having old and new units next to each other. Just imagine Jim Raynor head of a poor group of rebels. And he only get's his hands on whatever he can salvage/afford. A lot of that being the older technology. And his rebels might have captured the occasional modern unit too hence the new and old fighting alongside each other. And also Jim's marines might be using the old power suits hence less HP but the Dominion or whoever might be using the new marine power suits hence the more HP. I think the campaign all depends on which factions have created/adopted the new technologies and which have not. And another reason I think why the Wraith is not returned is in SC1 most units were all round units. Good at one thing, but can do others at a stretch for a while before dying. The wraith great at stealth attacking or ok at air to air attacks. But it's Air to ground purely sucks. But in SC2 Blizzard want more of a hard counter and less all round units strategy. The viking is either air to air or ground to ground. But now it's vulnerable to ground only attacking units like the zergling making it a much more situational unit, more hard counter and less all round. The wraith's air to ground lazer sucked but at least it had one. So zerglings could not kill it. If you want to fight the zerglings on the other hand you put yourself at risk from the zerglings. Another unit, the Banshee had awesome Air to ground attack. But no air to air like the Viking. So it rocks vs the ground units but air to air kill it unchallenged. The viking on the other hand can be ised in all secnarios. Sure some scenarios are more suicidal than others but they can at least try to be worked. Basically the differing focus on the SC2 and SC1 also made the wraith not appear in the multiplayer of SC2.
yeah, wraith ATG sucked. it was mostly an ATA fighter, and the viking does a better job of that than the wraith does. still, it would have been nice if the viking's GTG fighting was better, since it is supposedly a goliath replacement.
we're not talking in-game statistics here, but lorewise. I assume it's because the terran unspecialized fighters really don't stand the slightest hint of a chance against the other races(clouds of infectous fungi, and shielded ships with anti-matter missiles). Or it could be fuel efficiency
If we assume the Viking was developed lore-wise before the Banshee as it's initially buildable through the Starport, though it's not needed for this reasoning: Imagine you have the Wraith in your army, then someone develops the Viking, which is supposedly better at attacking air essentially taking the Wraith's role as air-defender. Then the Wraith get a somewhat akward role as a sort of laser-bomber before it's finally replaced by the Banshee. Also since the Banshee doesn't look so high-tech, it's neither a transformer nor a space-ship it might even be some disgarded mercenary-vehicle that was just put into use because even its primitive design worked better than the Wraith to hit ground units. At the same time, since the Wraith is some space-ship it might even in SC1 had some awkward role as a space-fighter they're using as a chapper.
To Fake ID: I agree. The Wraith became the Banshee I think. They sacrificed ATA so the ATG could be much better. Or it is more likely, they sacrificed the poor ATG laser. Then modified the ATA missles to attack ATG instead. So it became more an fast hit and run arial bomber. Which is rather unique, as most bombers in history were big heavy and slow. Just like the Goliath became the Viking. The Goliath could not be hit by ATA only fighters. And maybe the Terrans thought, giving the new Viking an arial transformation was worth giving up that ATA immumity, as vikings need to go airborne to attack ATA fighters. As the Viking is ATA and GTG. Maximum mobility but at a cost of maximum vulnrability.