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.
Let's give each other due
respect, and approach discussion rigorously. We're all peers and equals here as fellow hobbyists, with common interests in the acquisition and use of fountain pens, but not necessarily any shared values, and no obligation to offer each other moral support for one's narrative or position.
Don't think 'cos I understand, I care
Don't think 'cos I'm talking, we're friends
—'6 Underground' by Sneaker Pimps