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I'm just going to touch on the points I bolded if you have no issues with that.
I don't believe that Bitcoin in its current state is anything close to predictable or stable when it comes to measuring up as a currency, but it is definitely secure when it comes to international trade. I don't know if it is really able to be anything close to a global reserve currency yet, however in the future it might be something to take note on. It would be interesting to see the $30T evaporate from economic activity due to Bitcoin becoming the most prominent medium, however.
I think that you bring up an interesting point about alts but you do miss the point that even though they are more centralized, they do pose an interesting medium (especially Ethereum) for performing different business-related transactions. They're not something to scoff at but they're not irrelevant either. They are more tantalizing to investors, but that can mean a lot.
I'm curious as to how you think alts are "centralized" by default? Especially compared to Bitcoin, where miners have obviously become more centralized than ever. We are currently seeing miners right now have a war of blocksize ideology as I type.
I do agree though that alts do bring more to the table and allows people more freedom to try stuff theirselves, rather than just wait for the all might bitcoin core team to fork or do something that helps fix any problem whatsoever.
I don't believe that most alts are centralized by default, but Ethereum is a very centralized platform compared to a majority of other altcoins. That's more or less what I'm referencing, and it is less about alts in general. The dev team for ETH is basically the only one and they're very active when it comes to developing their crypto, and that is a good thing as it ensures there is a constant better quality of product, but it does generate some discontent with the freedom-heavy side of the community.
Yes, there is mining power that is centralized compared to other cryptos, and I find that to be an issue, but it's more just the dev team I'm referencing (think BU or Core had complete control). Anyone can become a miner (with a large investment) and have their own say, you can't be a dev team and get a vote.
Glad to see that you also saw my last point how other alts allow for a larger variety of concepts to be presented. Most people tunnel-vision on one point.