When to change a cartridge?

I currently own an EMT HD006 cartridge that cost about $1900. That's as much as I've ever paid for a cartridge. Previously I was in thousand-dollar territory. I can't imagine spending $5K for something that's assured to wear out. I play my turntable (VPI Prime Signature 21) every day for at least a few hourse. I guage that I put about a thousand hours a year on my cartridge, which is now at about 1500 hours.

I have read forums in which people talk about putting their cartridge under a microscope every few months. I don't own a microscope and I wouldn't know what to look for if I did. After reading forums in which people talk about cartridges wearing out before the manufacturer's recommended hours, I began to hear my cartridge slowly declining. I thought perhaps the attacks weren't as crisp.

I called my online dealer to discuss replacing mine, telling him that I thought I heard deterioration in the cartridge's sound. He said it doesn't work that way. I will know when my cartridge is ready to be changed. It will not be subtle. Often the suspension collapses. 

My reaction was that a dealer wouldn't talk me out of spending about $2K unless that expense was foolish. So, I am still playing my EMT HD006 and not worrying about subtle changes as it wears down. The dealer said it might be fine for up to 3,000 hours.

I'm curious to know what other people do about their cartridges. Wait for the suspension to collapse and the thing sounds terrible, or monitor it more closely and perhaps even change the cartridge before the manufacturer's recommendation?