You're talking like they lost all of their colonies and the systems they were located on were utterly annihilated; this is purely false. They were invaded by Covenant forces, and in the case of a particularly difficult or important planet, the major centers of civilian or military activity were glassed. Taking back their colonies would be a simple task. The real problem was the massive loss of life, but considering most of them were colonies as opposed to major shipyards or military bases, in the long run that isn't much of a hit in regards to what we're talking about.
What you said about the Iowa class is well and truly false, mostly because the time difference between it and current day compared to the time difference between the PoA and 2557 is a lot less, and the 70 years between 1940 and 2010 saw far, far, FAR more technological advance than 2510 and 2550 did. It's comparing apples to oranges.
In regards to what you said about building a fleet of technologically advanced and expensive ships; yeah, I can agree on that, for the most part. Without going into too much economic mumbo-jumbo, while it's not very likely its still within the realm of possibility that they would be built. Essentially, if the new Halo trilogy had taken place about 10-15 years later, it would be far less sketchy.
About the Infinity being a giant tube; lol. Are you trying to imply that ANY UNSC ships from Bungie's games ever looked all that good? You must be joking, surely. The Halcyon is a box, the Marathon is a box with a bite taken out of it, and the Paris/Charon/etc style of frigate is a tube with two boxes on the side. They're some of the least inspired ships in Sci-Fi. They
never looked all that good.
Don't get me wrong; I fucking despise 343 and "Halo" 4 & 5 with all of my being. Their lore additions are pretty much straight trash, and the games were liquefied shite. But they did get some of the stuff right.
Well the Marathon's were just as expensive pretty much. They were bigger and iirc had a lot of technical problems and needed lots of maintenance. They were built at a time where anything would be better than nothing. Also the Keyes class as far as we know is really only visually similar to the Halcyons and it's smaller size makes them easier to produce since your conyard can also be smaller.
It's referred to as the Autumn-class, and it's actually 300 meters longer than a Halcyon. The Marathons were also a little cheaper, but not by much.
http://halo.wikia.com/wiki/Autumn-class_heavy_cruiser
http://halo.wikia.com/wiki/Marathon-class_heavy_cruiser