One of the most prominent complaints I've heard, since the release of Skyrim, is how easy it is to make gold and that there is not enough to spend it on.
Another complaint that I have found is that players cannot reallocate any of their perks that they regret choosing.

What would you think of a NPC who, for a (very high) price (20,000 gold or so...) could erase one perk and reassign it to another slot?

Easy and easy. Depends on how you play. I've never come over 6000 gold, and that was saving hard to buy Breezehome.
That aside, could be a good option to have the possibility to fix how you have arranged your perks

A problem with reallocating perks is that the replay value of the game would go down tremondously.

Another problem is that one could max out all the crafting skills, create awesome gear, then remove those perks and dump them into all the combat or magic skills.

We can't "unlearn" what we already know.

Maybe a quest in a future DLC/Expansion that lets you reallocate your perks for ONE time as a quest reward would seem more feasible.
As for spending money, i spend mine on Ebony Ingots i like to collect them haha

I always oppose such things, for exactly this reason. I can 100% guarantee that if it ever gets implemented, that exact behavior will become standard when optimizing a character.

There are some games for which it's actually a good thing to have, however Skyrim isn't really one of them since there's a lot more margin for error when allocating perks. Now, if perk picks were significantly scarcer than they currently are then I could see having such a feature, but as things are it's really not warranted.

Reallocating perks would be the final nail in the RPG coffin for Skyrim.

Oh wow... That's true, I never thought of that. Yeah, because of this reallocating perks would definitely be a bad idea.

And the fact that it would pretty much destroy the replay value is another big issue.

100,000 gold for 1 unallocated perk, that would be something it would be easier to start a new game. Nah, bad idea.