The mempool on your own node does not mean anything to which transactions will get confirmed, unless you are mining as an entity that decides which transactions get confirmed (you are solo mining, or are running a pool).
It depends on sighashes. If you can join 500 transactions with one satoshi fee, to reach a single transaction with 500 satoshi fee, then you don't have to be a miner, you can just be a validator. It is ongoing, and there are practical examples, where you can reach fee rates below 1 sat/vB, or where you can even earn something, see testnet3 transaction: 99459ff5ce058067ed87b99f326305768444068a5659dce5ea5f126bfd4b0bda.
A pool's nodes could potentially keep these types of transactions, however, they have little reason to confirm these transactions, even if the mempool has no fee-paying transactions because there is an incremental cost for including an additional transaction by way of having a slightly higher chance the block will get orphaned.
Keep calm, today 1 sat/vB means "no priority", and anything below that means "joke priority", so all users should be aware that their transaction can be rejected. But so far so good, in the past, it was normal to pay 0.01 BTC as a fee. But it is sad that the community removed free transactions entirely from the "reasonable defaults". However, regular fees can now reach values like 100 satoshis. In the future, I think we could reach even single satoshis on mainnet, if the whole traffic will jump into sidechains or Lightning Network. It is a matter of time, historically, fees were decreasing, when it comes to the number of satoshis.