Light (or ultralight) backpacking as a metaphor for FIRE is totally applicable. The same process of optimization applies. Just as with finance, there is also the infinite continuum of choices and outcomes.
In hiking, I phrase it as balancing the desired comfort at camp versus on the trail. A person who likes camping, for whom hiking is something you just have to do to get to the camp site, will optimize toward a heavier pack that provides for the more enjoyable camp experience - bigger tent, more comfortable sleeping pad, camp luxuries like a French Press, etc. For the opposite person, one for whom camping is merely the act of sleeping in between hiking sessions, the minimal camp gear to provide the required shelter, food, etc is all they should take. Of course, there is an infinite series of options between the two, and it all depends on the individual to figure out what is right for THEM, in THEIR unique circumstances.
First off, let me say that light / ultra light is not necessarily more expensive. Like everything in life, it depends. For example, the best light weight rain gear in my opinion is a poncho. Not the 3 dollar kind, but a solid coated nylon that REI has for about $30. This is lighter, less expensive and far more effective than even the cheapest "breathable" rain jacket. As a bonus, it also covers your pack, keeping your gear dry as well. Will a person spend more on a high quality down sleeping bag? Yes....but in the context of "buy it for life" the boots as metaphor for poor / cheap, frugal / value context, spending a bit extra to buy a high quality down bag the first time will provide a high quality product that, if cared for, will last decades and be the better overall value than a crap bag that needs to be replaced frequently. The bag I carried on my PCT thru hike in the mid 00's is still going strong as an example of the value proposition of quality.
OK, on to the light hiking / FIRE metaphor:
Both are impacted by positive feedback loops. In FIRE, every dollar not spent moves from the spending side to the savings side of the savings rate equation. As we all know from Jacob at ERE and the Shockingly Simple Math, it's all about the savings RATE. Eliminating $100 in monthly spending both reduces the need for savings to generate the income to support said $100 of spending AND adds to the savings at the same time.
I use this example to illustrate this effect to the unwashed masses: Say you're trying to save up that 6 months of emergency fund. If you save 10%, how long will it take (neglect interest / investment gains to simplify the math)? Well, for each time increment, I'm spending $9 and saving $1 (the 10% savings, therefore 90% spending), therefore it will take 9 time increments to save up 1 time increment of money. Therefore it will take 9 increments x 6 months = 54 months, or 4 1/2 years to save up a 6 month emergency fund. If instead you figure out how to become more efficient on the spending (lower cost cell plan, cut back / eliminate cable, right size the housing, etc) and can save 20%, how long will it take? It only takes 4 time units to save 1 time increment of expenses (spend 80 / save 20 = 4 / 1). 4 increments x 6 months = 24 months or 2 years ....less than half the time, but only by saving 2x as much. Note the non-linear, positive feedback effect.
The same positive feedback with non-linear improvements occurs in hiking gear. The heavier ones gear is, the heavier it needs to be. The corollary is also true - light gear enables light gear.
Think of it this way - if you choose a bulky heavy tent (bulk and weight are usually correlated in hiking gear), sleeping bag, thick heavy sleeping pad, white gas stove, big multi-tool of a knife, stainless steel pot, rain parka, pack cover, etc, etc, etc you need a big bulky heavy pack to haul all that shit, adding still more weight. With all that, you'll also need additional foot and ankle support....heavier footwear. And be moving slower with all that weight....so need more food and water, further adding to the load, further slowing you down.
OTOH, a compact light shelter, high grade compact packing sleeping bag, light foam pad, soda can or light cartridge stove, titanium or aluminum right sized cook pot, poncho, etc ENABLES a smaller, lighter weight pack. With the lighter load on ones back, a hiker can get by with lighter, more comfortable and less fatiguing foot wear. All of this enables a hiker to, with equal effort, cover more miles in a day thereby reducing food load for the planned hike distance. Given distance between water sources are covered more quickly, thereby reducing the amount of water that must be carried, further reducing carried weight.
Note the positive feed backs - compact and light gear ENABLES other compact and light gear. Heavy and bulky REQUIRES other heavy or bulky gear.
FIRE: The more you spend (lower savings rate) the more you must save / invest to have enough income to support the spending and the exponentially longer it will take due to the lower savings rate. The less you spend, the exponentially faster you can get to FI due to the combination of less required savings / investments to support the low spending and the higher savings rate.
