Why do people think that because max block size 20Mb = ALL blocks will be 20Mb?
can someone please explain this?
Because some people are idiots or have their own agenda. It's that easy.
Nobody has said "ALL blocks will be 20Mb."
But what if they were? Nobody knows, so we should gather enough data to make informed predictions and preparations.
Right now, there are so few full 1MB blocks that we have no idea how the system would react, especially to much larger ones.
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If most 1MB blocks aren't full, to even act like all or most 20MB blocks would be full just shows how full of shit you are(or could be, I'm not calling you full of shit, don't take offense please). If most 1MB blocks aren't full, why not go to 10MB first? There is logic in code, so someone who knows bit about bitcoins code and the new 20MB code CAN predict how the fork to 20MB will react.
This isn't religion. This is very simply pretty predictable stuff.