Let's agree to disagree.

: Halo has always been a bit more light-hearted in some
: respects, and that ties into it (along with steampunk and pizza skins.)
: It'll always make more sense to me than the massive number of MJOLNIR Mk V
: armor variants we got in Reach.

The problem is that I could envision most of Reach's armor sets existing. There were a few with thin justifications, like the helmet specifically designed for forward observers, but at least only the helmets had an obscene number of variations.

: With Infinity, though, I feel like the largely unmentioned implication with
: the project is that it was being built and designed for a universe where
: humanity had lost—an Ark rather than a direct weapon of war. As you point
: out aside from the discovery of the Halos and Trevelyan's engineers it
: would have missed any sort of major deployment in the war by years, rather
: than months, and at that point would still have been outclassed and
: outgunned by the might of the Covenant at its peak rather than the
: splintered and until only recently impoverished Covenant remnants. At the
: point it was constructed and until the end of the war, there didn't seem
: to be any solution to the problem of humanity being stuck on planets that
: they could not adequately defend.

This is an interesting question I've been thinking about, and as it turns out, an Ark is harder to keep running than Battlestar Galactica and Homeworld made it look. Warships exist and operate within the support structures that built them. They need the shipyards and drydocks that built them to do extensive repairs, and they need the factories to make the spare parts that keep them running. Those factories, in turn, require the factories and refineries and mines that supply them with materials, and they need the farms and the medical clinics that keep their workers alive and healthy.

Can you shrink that massive industrial supply chain down far enough to fit in a ship?

If you can't, can you shrink it down enough so that the industry you carry with you can maintain the ships needed to carry it?

Have we seen any indication that the UNSC's technology is sufficient to do this?

: A mobile city solves that, and goes
: along perfectly to me with ONI's "consider all options" stance.

"Half the Navy's budget for twenty years" is pretty expensive for a contingency plan.
I dunno. Would I like the concept of the Infinity if it wasn't built up to be so super-special? I'd like to think so.