I understand what you mean. The tap, in their first threads, are not the right diameter as it is designed to self-center. So if the hole is too shallow, when the tap bottoms out only a fraction of the hole has been threaded to accomodate the helicoil insert. That makes sense.
Regarding the initial investment of the tool vs replacement, if you think exclusively in economic terms and opportunity cost, you will always buy a replacement. But following that way of thinking, who would spend in a restoration the same -if not more- than in buying a new bike?
I like to think of it in a more philosophical way: I install my switches and find out I have an stumbling block with the mirror hole. I can always go to a bigger size, or buy different mirrors that can be tightened with a nut in the opposite side. All of them will solve the "problem" of how to fix mirrors on a bike, a problem that is completely unrelated with using the original ones, nor the original mount. You can always buy aftermarket mirrors with handlebar clamps.
When I stumbled onto that problem, all options considered, I thought that the most profitable was the repair. It may cost more in the short term -money- but I will surely learn something in the process, will have another tool in case I need it, and to my point of view, would be the option that not only would solve the problem, but would let me keep the original appearance.
I had the very same problem with a caliper in my Sanglas bike. I tried to fix it -in that case it was a broken bleed nipple that refused to get out-. Tried to drill it out, tried to tap a bigger hole with an oversize nipple, tried to fill the hole with epoxi and re-thread - no way, only a replacement did.
But hey, I tried....
