It may be inevitable that we keep spending on new bikes, upgrades and equipment as you said. Although I have resisted mightily, even I "splurged" on new Sora brakes as opposed to what I randomly had in my closet, when building the bike I didn't technically need for commuting. There is a relentless pull towards that slippery slope.
But, this being "commuting", it isn't strictly necessary. Just to balance this out, I'll say that the cost of bicycle commuting can approach a negligible amount, if you don't care about the bike or the ride and your only objective is travelling to work and back. Up until 2012 I think my total cost per mile was around two cents. Not that I'm recommending that, but just to put it into perspective.