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question is: will fees be exponentially higher again when it reaches 20K & mempools are full?

Should not be this way anymore, especially with Lightning network+Atomic swaps

LN is not Bitcoin & you eventually have to pay for it, just like the way you pay for internet to ISPs

LN is a protocol that aims to scale and accelerate blockchains one of the functions of LN are micropayments.

Understand, but what you're saying is not relevant to what I said....LN obviously scale Bitcoin "off-chain".......it's like you take your golds to the banks because they're heavy (high fees, "artificially") & banks give you paper money (fiat) so you can easily carry them around & pay in nickles.