Yes, you are quite right that you will often find this problem on older calipers and the PTFE (polytetrafluoroethylene) tape trick does seem to be helpful in these cases.
The tape is usually used in plumbing rather than motorcycle servicing, where it is generally a quick way to seal tapered BSP threads. I think the answer to your question is to do what you would do if you were plumbing, i.e. if the tape will not seal, wrap a bit more round until it does.
What you should find is that although wear will take place when you repeatedly slacken and tighten the bleed nipple, it will be the tape that gets it rather than the female thread in the caliper.
I don’t see why you should not use a Heli-Coil to repair the caliper thread: because the stainless steel insert is much stronger and harder than the Mazak caliper, it would not need to be as long to get the same effect, so in other words you just need to use an insert which does not reach all the way down to the bottom. You would need to put it in with thread locker to act as a sealant.
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I recently replaced the brake hoses, pads and bleed nipples on my 1999 Yamaha YZF-R1. I struggled to bleed the system due to fluid leaking between the threads of the nipple and caliper housing. Only after the application of a few winds of PTFE tape did it stop the leaking and then prevent air entering the system and allow the system to be bled normal.
Clearly, on a 19-year-old motorcycle the nipple have been tightened/untightened several times, which has resulted in a reduced fitting. A Heli-Coil kit wouldn’t fix the problem due to the tapered end of the nipple.
So, my question is what to do when PTFE tape no longer works?