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Hm well lets do a little math. For the sake of simplicity lets say monero transactions are the same size as bitcoin transactions. And that bitcoins 1mb blocks are what are necissary to keep sifficient decentralization at this time. Lets scale it at moores law over 20 years. In 20 years that is 13 doublings. Lets say half the people on earth use monero for day to day transactions. Lets say the average person makes about 5 transactions per day.
We would need (3.5billion*5/86400) ~200,000 transactions per second. Bitcoin does about 7tx/s. 13 doublings would put it at ~60,000tx/s. I mean it's in the same realm. Throw 2 more doublings in there and you would be > 200,000. IF moors law holds. Sooo maybe. Maybe it scales well enough in 20 years without some scaling breakthrough.
Yes, but it gets better. In the above scenario the blockchain would continue to grow at a linear rate, but there is no requirement for Moore's law to stop. So one would have an exponentially falling cost with a linea growth in blockchain size. This is what happened with credit / debit / payment cards. There was fall in the cost of "keeping the ledger" by many orders of magnitude between the tabulating machines of the 1940's and the mainframe computers of the 1970's. The cost of keeping the ledger kept falling even after market saturation.
The beauty of the Monero approach to the blocksize / scaling issue is that the market actually ends up setting the size of the blocks and the price of the blocks in response to the actual real cost of keeping the ledger. This is because the cost in terms of Monero of each block is actually set by the emission. So the market will end up setting the size of the blocks in terms of MB (or GB, TB etc) and cost of the blocks in terms of the purchasing power of a fixed amount, the tail emission, of XMR.
Edit: The above is accomplished without a pre determined blocksize limit (Bitcoin and Bitcoin type crypto currencies) or a fixed cost per MB (Ethereum and similar fixed fee models). In both of the Bitcoin or Ethereum cases changing the blocksize or the fees requires a hard fork.