The UNSC's tech is outdated for being 500 years in the future. Extremely outdated.
Humans now are already developing warp drives, active camouflage, exoskeletons, hydrogen fuel cell cars, nuclear reactors with zero emission, and more! While we certainly aren't even close to creating a 3 mile long super-ship, we should be far beyond that in, I dare say, 300 years! Maybe even less! Even a Spartan by then would be unimpressive!
For 500 years into the future, I'd be willing to wager that the Covenant would be little more than a bug to us. A nuisance.
By then, I would assume humanity would have things like... I've actually got nothing. What we could do by then is unthinkable. Maybe we can make stars go supernovae?
[i]What do you think?[/i]
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Just because all of those things are in development, does not mean they'll actually come to fruition, they might even prove to be unfeasible or impossible. Remember that some scientists in the 60s' thought we'd be living on the moon by now ? It's not just a case of advancing knowledge, resources are required as well. If it turns out that a warp drive takes an insane amount of resources that we simply don't have (which it is...For now.), then it can't be developed. Also, if for example, a plasma weapon took a lot of rare resources and time to manufacture, of course you would go with the much more convenient option of using abundant resources like metal for ammunition, guns, etc. I'm all for optimism about the advancement of knowledge and technology, but 500 years of advances can't possibly be predicted. Some day, 480 years from now, a group of scientists will find the server this thread sits in and laugh, just as they will with every other prediction made by scientists of this era.