[ About some transactions no miners pick up ]
If some transactions no miners pick up from their memory pools because of low fee compared to transaction size,
what will happen to these transactions in the future?
They will be forgotten about unless someone re-broadcasts them.
It is said that most miners drop transactions over 72 hours old from their memory pools.
There is no requirement for a miner to keep the transaction in their mempool for any amount of time. 72 hours does appear to be a popular common setting.
Is there any possibility that bitcoins used for these transactions will be lost?
If someone loses their private keys, then they will be unable to spend their bitcoins. The bitcoins
effectively don't move until the transaction is confirmed. Therefore, they are still accessible to the sender until the transaction confirms. Once the transaction is confirmed, the bitcoins are only accessible to the receiver. There is no intermediate place for the bitcoins. They are either available to the sender or to the receiver as long as they have their private keys.