Mempool

It is not true that you pay 1 s/B and the transaction will be Mempool

It is not true that you pay 1 s/B and the transaction will be confirmed.

For two reasons, the first one is that right now all transactions with less than 13.5 s/B are being discarded from the beginning and on the other hand you have the mempoolexpiry parameter that by default discards an unconfirmed transaction after 14 days.

It is also not true that if there is a node that keeps that transaction in the mempool because it has a higher mempoolexpiry configured, it will be confirmed, because the mempool is not retransmitted through the nodes, only the new transactions and the miner should feed from that mempool with a higher expiration time, therefore and in practice your transaction will be discarded after 14 days.