So you're picking some arbitrary level of units sold to define when a market is established? What is that number and how did you arrive at it?
I had an Archos Jukebox that I bought a few months before the iPod was introduced. It sucked. I worked at the time at a company with several thousand well-educated tech-using employees and I was the only one who had a hard-drive based mp3 player. That's not a market, that's a curiosity. The biggest problems were shock-induced skipping, low battery life, no useful sync mechanism (I wound up writing my own playlist creator software), and abyssmal transfer times on USB1. The iPod solved all those, making a product with enough capacity to satisfy the general music listener (markedly superior to a CD's capacity), had a generous enough buffer to overcome the hard disk limitations, and with iTunes - a good sync mechanism. That's when the market formed.