Bitcoin for Point-of-Sale
Bitcoin is inherently ill-suited as a form of instant payment. This is due to transactions needing confirmations before they become reliable which even at best times can take between several minutes and an hour. Before a transaction is confirmed, it is merely a declaration of intent open to replacement by another transaction sending to a different recipient, i.e. a doublespend. This is in crass conflict with the requirements of a quick payment, e.g. at a super market with a line forming behind you or when purchasing hot food.
Value of transactions and fee market
Fees in Bitcoin are paid for the blockchain space that a transaction occupies, not relative to the amount of value that is transferred. As the data-wise smallest possible transaction could transfer $1 or $1,000,000 of value, it is obvious that a more valuable transaction can afford to pay a higher fee for the same blockspace while remaining economically viable. It is therefore inevitable that more valuable transactions will push out less valuable transactions in the competition for blockspace.
More miners and throughput
To discover a new block, a miner has to find a block template that fulfills the current difficulty level. This is a rebalancing system, if mining power increases, the difficulty will follow suit to slow down block discovery. Adding more mining power will only briefly create more capacity before the difficulty catches up.
2You are asking several different questions here, some of which are very subjective. Can you focus your question more narrowly? By the way, increasing the number of miners will not change anything, except temporarily - the rate of confirmed transactions is limited by block size, and the Bitcoin protocol keeps the block rate approximately constant over time. There are many questions about this issue on the site. – Nate Eldredge – 2017-05-29T16:02:04.160
Nate is correct by the way, you really shouldn't ask about multiple different topics in one question. ;) – Murch – 2017-05-29T20:36:45.670