The switch of counting Warp speed from regular numbers to an asymptotic cardinal counting system that approaches but never reaches 10 was started by an exceptionally bitter and sadistic Superluminal Physics professor at Starfleet Academy who had way too much sway in the scientific community for his or anyone else's good.
There are few, if any, asteroids left in the Sol system and many core Federation systems because while Starfleet won't mine on a world without contacting the local authorities, they can, will, and regularly do strip mine asteroids for everything they're worth. There are still a lot of asteroids left simply because there are a lot of asteroids.
Starfleet actually has dozens of designs for straight-up warships, they just don't use them often because they don't often face an enemy powerful enough to pose that much of a threat to Starfleet. And when they do, they usually get much of their shipbuilding wrecked, so they have to suffice with making Defiant-class destroyers by the hundreds. So designs for massive battleships and heavy cruisers packed with sweet phasery death sit gathering dust for years before either getting pulled out to smack the Dominion around, or shredded for becoming obsolete.