I like how this thread turned from "what do you regret buying" to "can we justify the cost of the hobby" lol.
I would like to share my opinion on this, because I actually left the hobby for a couple years due to the perceived sunken cost, and I regret that. I don't want the high cost of premium boards and keysets to scare away other hobbyists either. I missed out on a lot of cool innovation and opportunities, and the reason I left is not valid and never was.
Most people here are focusing on the high cost of GMK keycap sets. Sure, it is a big number to spend on keycaps, especially when you can get dyesub PBT caps for 40$ on amazon that do the job just as well. But this hobby isn't about pure functionality. Quality, brand recognition (factory and designer), rarity, hype factor, and customization (via kits) are some of the other relevant parameters. Also worth mentioning: it's not as if GMK sets have some unjustifiable inflated cost. They are high quality, double shot, injection molded plastics. Those molds are frickin expensive, and they don't last forever. The legends are sharp, and impossible to wear. The caps are thick.
But rather than trying to justify the high cost, I'd rather point out that GMK sets retain their value pretty well, which is clear by looking at the aftermarket. Unlike purchasing products which lose value the second you buy them (like a boat, a car, PC parts, etc), or products which lose their value completely (digital services, video games, food, or bike tires like someone else said), GMK sets are an investment which you can cash in at a later date, usually at-cost or at a profit (not encouraging flipping.)
The exception to this is obviously if your caps are damaged (your fault, or bad luck- manu defects are often addressed these days anyway), or too worn (in which case you must have gotten a lot of value out of them)
All of the above applies to any other high end caps like signature plastics, ePBT, Infinikey, etc. It also applies to keyboards, but even more so- because boards will often have less units and higher desirability.
The bottom line is that, for every keyset/keyboard GB, there are always new people joining the hobby, or veterans returning to the hobby, or even folk who were interested but couldn't get a spot, who are excited about products that are past GB stage, and are willing to pay GB cost or higher - in fact,
usually higher, in the case of premium limited-run products.
My theory is that this aftermarket value even improves over time, as the market rarity of your keyset/board increases, due to all the other units eventually finding their permanent homes. And I believe that further rounds of GBs/in-stock sales of something you're holding do not hurt this much, due to the speed at which the hobby is growing.
This healthy aftermarket is proof that most products in this hobby retain their value, and a lot of the time, even appreciate in value. For this reason, I think the hobby is really forgiving financially (if you can afford the upfront cost).
As for the hobby being inherently expensive, I would argue that if you want to try all the different layouts, keycap profiles, mounting types, case materials, etc.. then yeah, you're going to be spending a lot of money. And unfortunately, the best way to narrow down your personal tastes will be trying everything. Though I will say that there are very cheap ways to participate, like through hole DIY kits and cheap switches. Another option is: buy to try, sell to recoup, and buy the next thing. I did this for awhile and got to try a lot of stuff while losing very little (mostly my fault), and other times gaining.
Of course, there are better ways to invest your money if you're trying to build wealth. However, unlike some other hobbies, the money you invest is NOT lost. You are investing in learning and experiencing the variety of the hobby, and there's a good chance you can recoup that investment at a later date, maybe even at a profit. The high price tags make the hobby seem like a money burning machine, but you are getting products with lasting value for that money, not burning it.
There's no telling what the future holds for the health of the marketplace, but I would argue that
its obvious the hobby is only growing, and even as high-end boards become more commercialized and available at lower prices (looking at you, GMMK pro), that will never hurt (maybe even help) the value of rare hobbyist custom boards.
P.S. As for the guy with the 5k mountain bike in Peru example: I can cruise down it with a 200$ preowned Specialized MTB too, but its not going to be the same experience. That example works both ways.