There is no single mempool. Nodes evict a TX from their mempool if the mempool is full with higher fee rate TXs. As an example, if the node has a mempool of up to 300 MByte (the default, I think), and around 300 MByte are consumed by TXs worth 100 sat/vByte (or more), the TXs with 99 sat/vByte (and below) are dropped.

However, keep in mind that nodes re-broadcast their transactions. It might disappear from some nodes' mempools, but it might end up back in them once the fee rates are low again.

You can't be sure that the TX will not be mined, some "malicious" miner might still have access to the TX and include it despite the low fee rate. Be careful.