Buying Ghosts off the Arbiter would be an even better option, because it's something the UNSC can conceivably accomplish in the next 500 years.

: I mean, how hard can it be? They reverse engineer Huge Forerunner
: and Covenant systems aboard starships all the time. This should be pretty
: simple.

It shouldn't be simple! Forerunner and Covenant systems aren't easy to reverse engineer. Covenant systems are easier, because they're only 3,500 years of copying Forerunner tech ahead of us, compared to 6+ million years for the Forerunner. But, you know, if the Prophets were serious about maintaining their technocracy, they would have designed Covenant tech to be as hard to take apart and study as possible. And then there's the fact that the UNSC spent 50 years camped over a Forerunner city on Onyx without learning anything.

Covenant and Forerunner tech has only been 'pretty simple' to reverse engineer under 343i, and it amounts to ignoring common sense and prior canon so we can have GI JOE toys.