Simple situation. I make a transaction and set to low fee by mistake. Now i need to wait days for processing. Why i cant just cancel it and resubmit with higher fee? Ok, maybe this could create problems for services that dont check conformations on deposits. Another option. Why i cant set higher fee for transaction that already in mempool? As i know there are special wallets that allow to do so. Why this feature avaliable in only several not popular wallets?
As for me, option to make fee higher for mempool transactions is profitable for everyone. Miners will earn more. Recipient will recieve money faster. Sender will be happy. Why not?
First of all, you should check the Mempool size and prevailing fee levels before sending your coins. And secondly, even if you have put a lower fee for your transaction, you can accelerate it using ViaBTC or any other transaction accelerator. Personally, I don't believe that there should be an option to cancel unconfirmed transactions. Because if that is implemented, then there will be a lot of misuse. Getting a confirmation takes anywhere from 5 to 60 minutes. So many of the merchants accept payments, even before the confirmation. If this option is implemented, then the merchants may opt to wait until the transaction gets confirmed and it will result in wasting a lot of time for both the sides.