If "enough" (1 large or many small) miners are willing to fill 20 MB blocks of ~0 fee transactions,
Why would they be motivated to do so? If there's no motivation, then the rest of the argument does not stand.
then some bitcoin users will send ~0 fee transactions, and some miners that mine for transaction fees will stop mining, which weakens the security of the bitcoin network.
Why would these ones stop mining? They would still be profiting. I don't get it.
Increasing the block size limit reduces transaction fees. Eventually, miners are not mining for transaction fees, but mining as stakeholders to maintain the integrity of the blockchain. Only stakeholders mining doing PoW is like a really inefficient PoS.
What's the logic here? If miners are not mining for transactions fees, they are still mining for the subsidy. Either that or it would be a truly altruistic act. And even if it was, they are not proving their stakes at all. They are still proving they have done some work.