There is nothing contradictory. You can exceed all transactions globally (the accumulated credit card, bank transfers, paypal payments, cash transactions etc) but you can still be more inefficient than banks or visa.
I agree on the "we will never be able to process more than a centralized...", because, well.... p2p inefficiency.
No further comments.
Again, there is no "paradox" here.
The technology (of bitcoin) can accommodate the accumulated number of every single tx on the planet if you give hardware and network technology the necessary time to grow. Whether that time is 10 or 100 years, nobody has a clue. Perhaps some variables and constants in the software will need "enlargement" to be able to fit everything, if they are currently "downsized" for economy, but it can happen with near 100% certainty.
What is also 100% certain is that this can never be more efficient than a hierarchical system.
What VISA could do in 2015 due to being more efficient, Bitcoin will do in +X years due to the technology compensating for the p2p / decentralized induced inefficiency. Will VISA be able to transact MORE than Bitcoin at that point? YES. Will it matter? No. Why? Because human needs for transactions are finite. You don't jump from 3 purchases a day to 1000 purchases a day. So at some point VISA will be able to process, say, a trillion txs per day but there will be no trillion txs to process. So, for VISA, the technological progression is meaningless because it doesn't allow them to do more. Perhaps they could run their data centers cheaper and that's it. For bitcoin it's another issue altogether because technology unlocks the potential which is burdened by the inefficiency.