I wouldn't worry too much about the life of electret microphones. I have a pair of Harry Mendelssohn HM40 (they were the cheapest on the market when I bought them ten years ago) and have had no problems with the electret mic. As I am an instructor, they get a lot of use so I have had to re-wire the leads and plug a couple of times in ten years, but no problems with the microphone or earpieces.