I think it's because he wanted the reward to require interesting work to get? That seems to be his entire point, that upgrading your weapons could have been an interesting and involved process. One that has measurable stages of progress as you reforge your Excalibur to legendary status. Instead we got a slot machine that uses cash. And earning money efficiently in Terraria is an incredibly boring process.
This, mixed with a dislike of th'slot machine mechanic that only grows as the years pass -- rage not directed at Terraria specifically, but the mechanic in general. I can't think of a single example where it wasn't either boring or unhealthy, possibly both. One of the reasons I can't stand playing most MMOs for very long
Plus from what I remember, the psych behind it is pretty nasty (reason some people get addicted to slot machines, etc.). That doesn't help.
The top tier forgings aren't necessary, so you aren't being kept from anything without them, except the ability to say "I have the best mods possible for this weapon." If that's all you really wanted, why not just hack up one of those in the first place?
To see how much cash it took to happen I probably would just use the invhack to change the item's prefix now, though. Quicker, ha.
I'd already gone through the majority of the game's content without the best prefixes (without particularly good ones at all, really) and wanted to see how it went with 'em. Didn't want to spend weeks to months farming bosses for cash for -- as you note -- a fairly minor feature.
Ah, yes, the good old Skinner box. Press a button, maybe get a treat, maybe not. It's the same reason people get addicted to subjectively terrible games like Farmville - they get conditioned, psychologically, into pressing the button and hoping for a reward - even when
they aren't having any fun pushing the damn button. When you play a game and do something because you
have to, not because you
want, there's something wrong with that game.
Note: I know that in most single-player games, you
have to kill the X or save the Y to progress, but that's because you have to do it to progress further into the game - which you generally
want to do. What I'm talking about is when you're out having fun and you
have to check on your virtual crops and so forth, and not because you
actually want to, actually
enjoy mindlessly collecting the random drops from virtual wheat.