I was discussing this in another thread. It's pretty much assumed that Monero's blockchain will be huge if more people start using it and they will not be able to keep it from growing since they have choosen to go the dynamic blocksize route, and it's a tradeoff for the extra privacy technology for the blockchain to be biggger. The question to be asked here is: What's the point, if the blocksize is going to become so big that the nodes will be way too heavy for regular users and more and more node centralization will ensue until only specialized people will be able to run nodes?
Well, storage capacity grows over time. But frozen-in protocols don't. The non-mining nodes in bitcoin are worthless what concerns the decentralization of the network. They help somewhat in having a robust transmission network of transactions from users to miners, but any non-mining node that doesn't agree, can only do one thing: drop out of the system. A miner that doesn't agree, can mine according to his preferences, and fork off. But a non-mining node has nothing to say. If he doesn't propagate a transaction he doesn't like, another non-mining node will propagate it, and even in the end, the user can transmit his transaction directly to a mining node which is the only communication that matters.
So the number of non-mining nodes for bitcoin doesn't mean much: the centralized mining is much more of a concern.
It is nevertheless interesting for an individual to keep a non-mining node running, because it can help obfuscate his own transactions, and it can help him against fake information about the bitcoin block chain. But that's about it.
As such, even if there were 10 times less full but non-mining nodes on the bitcoin network, that wouldn't matter much.
However, what matters is the fact that there's a hard limit on the growth in the number of transactions that the block chain can handle. We're not even a factor 10 from that ultimate limit. Bitcoin has no order of magnitude growth in transactions left.
As most transactions are, for the moment "greater fool speculation" transactions, and not transactions used for an actual currency usage, there's still a lot of potential currency usage adoption possible in bitcoin, if the trading would go down. But there's a hard limit built in it.
As to the size of the block chain, even a chain that would grow with 1 TB a year only means a very modest investment for a node owner at this moment. You get a 1TB storage for the order of $100,-. So that's peanuts. 10 years ago, 1 TB of storage was much more expensive. We can presume that 10 years from now, 1 TB of storage will cost much less. So if 10 years from now, a block chain grows with a few 10 TB per year, wouldn't be a big issue for an individual wanting to have a full node either.
However, if the protocol has a frozen-in limit, then these technological improvements in storage don't mean anything.
A finite block size limit sounds like Bill Gates telling that 640 KB of RAM is more than enough for any personal computer (remember ?).
