Shouldn't we look at a frequently used pen the same way? How much per use, per day, per year, per generation?
You could, if that's how you prefer to look at pens that way. "We" could. I don't see any argument for "should", though, or what the benefit is in promoting, advocating or evangelising that approach so that it is more widespread and common in the global hobbyist community.
If adopting your perspective "makes" you feel you're getting better value (with or without being wiser in your purchase decision-making) out of your discretionary spending per dollar than the next consumer who takes a different approach, then it's more of a win to you and more power to you! My personal preference would be for a hundred (or a thousand, or more) of our peers to pursue whatever they want, throw their money in that direction, and in doing so change the landscape of the market and the priorities of the industry to suit themselves (directly or indirectly).
I did a lot of business analysis professionally for "bean-counters" in large corporations who were spending other people's (e.g. shareholders') money, and who needed to answer for costs per transaction, per hour, per agent, etc. Fountain pen acquisition and use is a hobby to me, and I don't see why I'd take the same clinical approach to how I spend my money and evaluate the "return" on my purchases in the name of personal enjoyment.
As always: 1. Implicit in everything and every instance I write on FPN is the invitation for you to judge me as a peer in the community. I think it's only due respect to take each other's written word in online discussion seriously and apply critical judgment. 2. I do not presume to judge for you what is right, correct or valid. If I make a claim, or refute a statement in a thread, and link to references and other information in support, I beseech you to review and consider those, and judge for yourself. I may be wrong. My position or say-so carries no more weight than anyone else's here, and external parties can speak for themselves with what they have published. 3. I endeavour to be frank and truthful in what I write, show or otherwise present when I relate my first-hand experiences that are not independently verifiable. If it is something you can test for yourself and see the results, I entreat you to do so.