>Thanks - I'll investigate this further

I'll give you a couple of references if you are interested. "Zero to Monero" (https://web.getmonero.org/library/Zero-to-Monero-2-0-0.pdf) is a good guide, but it may become outdated with the upcoming "FCMP++" upgrade. You might also be interested in Ring signatures and confidential transactions (CT) which were the original privacy improvements suggested for bitcoin.

>Any transactions more than a few blocks deep in the current blockchain will remain secure forever - even if the network shuts down.

No it won't. You get 51% attacks if the hash power of an attacker is high relative to miners. Bitcoin's security assumes that there are always miners so an attacker can't catch up. Monero (and a couple other cryptocurrencies) use a tail emission: The emission rate is constant, which sounds bad until you realize that it still means that inflation is always decreasing asymptotically as a % of the total supply. At the same time, you get a constant subsidy for miners.

>the transaction fees are independent of size

Every cryptocurrency I'm aware of uses this model. It's because the actual computational cost for a cryptocurrency network to process a transaction is independent of the amount of cryptocurrency transacted.

>the small blocks (~7 txn/sec cap) see to it that transaction fees increase with demand for transactions

You are assuming that there will be demand for bitcoin transactions. But if the purpose of bitcoin is to be a store of value and not a medium of exchange then this will not be the case. Not to mention you have all these layers like the lightning network that are ultimately designed to decrease the amount paid out to miners and redirect them to intermediaries instead. Clearly the total mining reward from fees increases as the transaction throughput increases, and vice versa: eventually fees just get so high that they are prohibitive to certain classes of commerce.

Why is bitcoin a store of value, is it a useful instrument for trade? No, in fact the high fees are somewhat prohibitive to its use as a store of value. Bitcoin's value as a "store of value" is predicated on circular reasoning. You will eventually learn that self-fulfilling prophecies like these are also self-unfulfilling prophecies by the same manner.

Imagine that there is another cryptocurrency that becomes an effective instrument for trade at scale (monero wouldn't work because of its large transaction sizes, but maybe Bitcoin Cash or Litecoin or something) and at the same time is sound enough to provide a "store of value". This new cryptocurrency would out-compete Bitcoin in usefulness, and Bitcoin would no longer be an effective "store of value".

>If regular people can't afford to onboard, so be it. Rich people, large companies and countries will be able to, and it's this large injection of wealth that will make it extremely valuable, rather than piggy banks etc.

It would represent a wealth transfer to the lower class. These large institutions would be investing in an asset that is ultimately valuable "because it is".

As a thought experiment: If one person bought up all the bitcoin in the world and monopolized it, would it be valuable or worthless? The answer is clearly "worthless".

It's not the same as gold in this sense. If you monopolized gold somehow, you could set a much higher price based on its intrinsic usefulness as a material. Cryptocurrencies, like real currencies are only useful insofar as you can use them as an instrument of trade. There is a network effect involved.