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You are only looking at technical aspects and missing the political aspect.
Nodes don't do politics. People do, and politics is a lot larger with a lot more moving parts than just node operation.
> full nodes protect the user from the change of any properties of Bitcoin which they do not agree with.
Full nodes protect from nothing if the chain they attempt to use is nonfunctional.
> The ability to retain this power for users is of prime importance and is arguably what gives Bitcoin most of it's value
> Any increase in the cost to run a full node is an increase in cost to maintain monetary sovereignty
This power is far more complicated than just nodes. You're implying that node operation == political participation. Node operation is only a very small part of the grand picture of the bitcoin balance of power.
> The ability for a user to run a node is what keeps the miners honest and prevents them from rewriting any of Bitcoin's rules.
No, it isn't. Nodes disagreeing with miners is necessary but not sufficient to prevent that. Nodes can't utilize a nonfunctional chain, nor can they utilize a coin with no exchanges.
> What makes Bitcoin uncensorable
Only two things - 1. Node propagation being strong enough that a target node can't be surrounded by attacker nodes (or so that attacker nodes can't segment honest nodes), and 2. Miners being distributed in enough countries and locations to avoid any single outside attacker group from having enough leverage to prevent transaction inclusion, and miners also having enough incentives(philosophical or economic) to refuse to collude towards transaction exclusion.
Being able to run a node yourself has no real effect on either of the two. Either we have enough nodes that an attacker can't segment the network or we don't.
> What gives confidence that the 21 million limit will be upheld
What you're describing would result in a fork war. The opposition to this would widespread and preventing an attempt relies upon mutual destruction. If users refused to get on board, exchanges would follow users. If miners refused to get on board, the attempt would be equally dead in the water. It would require a majority of users, businesses and miners to change the limit; Doing so without an overwhelming majority(90% at least) would still result in a contentious fork that punished both sides(in price, confidence, adoption, and possibly chain or node attacks) for refusing to agree.
Nodes have absolutely no say in the matter if they can't segment the network, and even if they could their impact could be repaired. Users != Nodes.
> What makes transactions irreversible
Err, this makes me worry that you don't understand how blockchains work... This is because miners are severely punished for attempting to mine on anything but the longest chain. Nodes have absolutely no say in the matter, they always follow the longest chain unless a hardfork was applied. If the hardfork has overwhelming consensus, i.e. stopping a 51% attack, then the attack would be handled. If the hardfork did not have overwhelming consensus it would result in another fork war requiring users, businesses, and miners to actively decide which to support and how, and once again would involve mutual destruction on both forks.
Nodes don't decide any of these things. Nodes follow the longest chain, and have no practical choices in the matter. Users not running nodes doesn't diminish their power - Mutual destruction comes from the market forces on the exchanges, and they could give a rats ass whether you run a node or not.
> The market is not storing 10s of billions of dollars in Bitcoin despite all it's risks because it is useful for everyday transactions, that is a solved problem in every part of the world (Cash/Visa/etc..).
This is just the "bitcoin is gold" argument. Bitcoin is not gold. For someone not already a believer, Bitcoin is a risky, speculative investment into a promising future technology, whereas gold is a stable physical asset with 4,000 years of acceptance history that has the same value in nearly every city on the planet. Bitcoin is difficult to purchase and difficult to find someone to exchange for goods or services. Literally the only reason we have 10s of billions of dollars of value is because speculation, which includes nearly all Bitcoin users/holders and almost all businesses and miners. While Bitcoin borrows useful features from gold, it has more possible uses, including uses that were never possible before Bitcoin existed, and we believe that gives it huge potential.
The ability of other systems to do transactions, like visa or cash, come with the limitations of those systems. Bitcoin was designed to break those limitations and STILL provide the ability to do transactions. We might all agree Bitcoin isn't going to ever solve the microtransaction problem, at least not on-chain, but saying Bitcoin doesn't need utility is just foolish. Gold doesn't need utility, gold has 4,000 years of history. We don't.
> Even if we fork to 2MB, 5MB, 10MB. It is irrelevant in the larger picture, transaction capacity will still be too low for global usage in the medium-long term.
Which is why it needs to be a formula or a continuous process, not a single number.
> Even if it fails to live up to the hype, you should not discount the market innovating solutions when there is money to be made.
That's like saying it would be better to do nothing so someone else solves our problem for us than it would be for us to do what we can to solve it ourselves. Someone else solving our problem may very well be Ethereum, and "solving it for us" is pulling Bitcoin investments, users and nodes away into Ethereum.
> The additional capacity from blocksize increases are linear improvements with very large systemic costs compared with the userbase and usage which is growing exponentially.
The capacity increases do not have to be linear. The increases in utility are linear with blocksize increases, but so are the costs. There's no reason those blocksize increases can't be tied to or related to usage increases, so long as the concerns about having too few nodes (or too few fees) for security are handled.