Economics

…in terms of capitalism, this is horrible. How does an economy grow if the supply of money remains unchanged?

The economy will be able to grow just fine and normally: by producing an ever-increasing value of goods and services. We have good historical precedent for this: countries and empires that existed before the invention of paper currency. Or the US when it was on the gold standard. In a fixed supply currency economy, when the economy grows, the money becomes more valuable. A dollar is essentially a "share" of the entire US economy. If a company you have a share in becomes more valuable, what happens to your share of it? It becomes more valuable too. In a fixed currency economy, when the economy grows, the benefit is shared with all economic participants according to how much currency they have.

In an inflationary supply economy, when the economy grows, if supply inflation outpaces growth (which it is does) then the currency becomes less valuable. Meanwhile, value transfers to the people to printed the additional currency supply and whomever they decided to give it to. This is what happens every time we do bank bailouts. The 99% end up paying for the investment mistakes of the 1% and the flaws of fractional reserve banking.

Imagine how the world might look different if governments couldn't print money at a whim. If they wanted to fund wars, they would have to raise taxes. That would not be popular. Might we end up with a more peaceful planet if that were the case?

If you expect your currency to generally appreciate in value, somebody has to really convince you to part with it. Might our goods be built to last? Be more repairable? More sustainable?. Perhaps, one can only speculate. Having money that loses its value over time forces people to spend it as quickly as possible. The faster your money depreciates, the more wreckless you will be with spending because even getting your 15th blender is worth more than your 100 Turkish Lira will be in a month. A currency which loses value by design fuels needless consumption/production. On a planet with finite resources, this is a questionable incentive to have in our economic system.

• Pools became more centralized for reasons of economic and energy efficiency. Pools themselves do not actually have any hashpower, they simply relay the results of mining to the rest of the network.
• They have never performed a 51% attack because every incentive is aligned against doing so.
• Pools have been getting more decentralized in recent years due to protocol upgrades.
• What you can do with a 51% attack is extremely limited. You can't spend anybody else's money and you can't print BTC which isn't meant to be printed. The only thing you can theoretically do is a double-spend, and pulling that off is basically impossible due to the amount of value you need to transfer extremely quickly, nobody being able to hand you a billion dollars of oil in 30 minutes, and needing to wait for multiple block confirmations for a purchase that large.
• Anybody in any country with electricity can mine Bitcoin. If you want to be real efficient about it, you spend a few hundred to a few thousand USD for an ASIC
• Energy is the most equitably distributed resource in the world. It falls from the sky. It runs in every river and gust of wind. It is in the crust as uranium.

Also, why is having a set unchanging hard limit on total supply that important?

Because otherwise somebody else is printing that currency and transferring value from your currency to whatever they want to do with it. And if we give an authority the ability to print currency, they may be a great custodian of that trust until they aren't. Historically, for the most part, government haven't been, they've just been the best solution so far to the problem of who to trust with the currency supply because they were the most stable institutions humanity had ever built.

Bitcoin's value relative to the dollar is unstable, yes, but it's more stable than many other currencies out there. And it's stable relative to its own supply. 1 BTC will always be 1 BTC. No fiat currency can make that promise. Stability improves with scale, the more people use it, the more stable it becomes, same as any other currency.

People don't use USD because the US will bomb them if they don't. They use USD because they trust the fiscal policies of its issuer and many places accept USD (network effect). Not every country likes using USD because we use it as a tool of imperial control over others, so sometimes they choose other currencies. Not often though, because they don't trust the issuers of those other currencies.

Bitcoin has fiscal policy that is backed by math and physics instead of the whims of a national government and its network effect grows more each year. Only time will tell if that can result in Bitcoin becoming a global reserve currency.