To the miners won't change much if the fee comes from mining 2 transactions or one,
The miners only care about how much fee is paid per block weight unit.
The fact is that right now the system is able to process 3 transactions per second,
The fact is that SegWit has increased that number. You are quoting an old and outdated number.
My guess is the miners would be extremely happy to be able to have a system to process more than 3 transactions per second and win a lower fee from each of them
Your guess is provably be wrong.
Bitcoin Cash already exists as a protocol. It allows up to 8 MB of block space, which increases the quantity of average size transactions from 3 per second to 24 per second. And yet, the majority of miners are choosing to mine on the Bitcoin Blockchain where they get higher fees, but less transactions.
What's that number now? it's enough to support the transaction needed by the world economy?
Don't you think the miners are choosing to mine bitcoin because of the value of the coin? what's going to happen if the value of bitcoin cash raises?
and still, I haven't heard a convincing motivation to not to improve a system that cannot support the number of transactions that needs to support