If there are less miners, the transaction will be as slow as you can imagine. Also this will affect the users, they might switch to other alternative coins for fast transactions. So in this scenario, less demand, low price.
But if the miners will just rely on transaction fee of course this will not be enough for them versus in what their getting in block rewards as a result of this, over time miners will not be interested to this and will quit. The lesser the miners ther slower will be the confirmation of transaction.
The Bitcoin protocol is designed to produce one block every 10 minutes. If the number of miners goes down, the difficulty will automatically adjust to keep the block time approximately the same. Thus, transaction speed will not change.
Because it becomes exponentially difficult, then there is no possibility to obtain the last bitcoin of the 21,000,000 of the bitcoins. So computation will continue forever, and give exponentially difficult results. This will give the possibility for the bitcoin to exist forever. The last bitcoin, or the last part of it will be never obtained.
Because bitcoins are measured in Satoshis (.00000001 BTC), as an integer value, the block reward will actually go to zero, 4 years after it goes to 1, because 1 divided by 2 is 0, when using integer arithmetic.