The OP has posted a pretty general question.
.58 is not "dead" by any means. Either in smoothbore or rifle.
Rifles . . if it was dead, N-SSA would be in existence as most rifled muskets are .58 caliber - I've shot rifled muskets for 50 years. But . . those take a "minie ball".
If you are talking round ball . . . I have know quite a few who have 58 caliber smoothbore trade guns. Very versatile for both round ball or shot.
If you're talking rifle . . I also have know quite a few who have 58 caliber round ball rifles. Not only Hawken style but full stock Kentucky style rifles as well. While I've owned a few "production" guns, most of mine have been custom built by myself over the years. And . . you'll see a number of customer built rifles in that caliber.
Carbine? Again . . what are you referring to? A military style such as the Parker Hale .58 caliber rifles or a "civilian" style which is equivalent to a "short rifle"?
As far as I'm concerned . . no caliber is "obsolete" . . . especially in muzzle loading. All you need is a mold of the right size and you are all set to go.
If you are hunting . . . you choose the rifle/caliber that is appropriate for the game you are hunting. 58 caliber? An excellent choice for large game . . deer, small bear, etc. But if you are going to hunt squirrels with it . . yes . . . I guess you could say it's "obsolete" . . . unless you want to shoot 'em and clean 'em at the same time.But then again, I wouldn't be choosing my 36 caliber to hunt bear either . . . . doing that might make "ME" obsolete!