In the 1950's, seeds were exposed to Cobalt-60 and then sold for people to plant and grow into whatever they were mutated into. It was called "atomic gardening". https://en.wikipedia.org/wiki/Atomic_gardening
"...the security could be updated but I do not see how this can be done in practice without a central authority"
The same way the fork was fixed and how any upgrade happens with Bitcoin. An idea is proposed, a small group of developers work on it, the mining pool operators decide to implement it or not, then all the individual miners vote on it by deciding which mining pool to mine for. Basically any change has to have 51% agreement from the miners.
"Ironically, for what is suppose to be a highly decentralized currency, the collapse was avoided because the system was "controlled by the will of a very small number of people – particularly, the operators of mining pools" who effectively agreed which block chain to support."
The avoided collapse only appears to be a centralization of power because it was a good decision. I'm defining good decision here as one that increases the value of a bitcoin. The majority of miners were using the new software, the majority of businesses were using the old software. If nothing was done, bitcoin mining for everyone with the new version would have kept plugging along fine, but all the businesses that had to re-install or rewrite their old Bitcoin software would have had to shut down for many hours or days.
The majority of mining pool operators decided that it was better for the longterm growth of Bitcoin to give up the bitcoins that they mined on the new chain and minimize the downtime of Bitcoin businesses rather than keep the small amount of bitcoins and have Bitcoin suffer bad publicity in the coming week. This is a very important point. Every individual miner had a choice of what pool to use. When they found out about the change, they could have just as easily flipped a switch and started mining for a pool that decided to keep the bitcoins. That didn't happen. The Bitcoin miners/holders aren't reacting only to what gives them the most bitcoins today. They are thinking long term and are doing what they think will give their mined bitcoins the most value in the future.
Bitcoin is sort of like the federal government and the altcoin experiments are like state governments. If there's a really good idea that an altcoin comes up with, Bitcoin can integrate it by a 51% vote. If an altcoin experiment uses SHA-512 successfully and is gaining value over Bitcoin because of it, bitcoin can integrate that feature by a 51% vote. If an altcoin has a desirable feature, then bitcoin might be able to integrate it and enjoy the benefits of the feature + the network effect to stay relevant.