Are you talking about a 51% attack?
You should quote a certain part of my message, because it's too long to understand where that question refers to. No, I'm definitely not talking about a 51% attack.I'm not sure what you mean by no bitcoin. Do you mean it would morph into something else or cease to exist?
That's a discussion that broaches a lot of people. You'll only read opinions since there is no one that can define what is Bitcoin. I personally define "Bitcoin" a computer program that is compatible with those consensus rules. Any other program that defies them should not be called "Bitcoin".Do you know which cryptocurrency has the lowest transaction fees?
It depends on what kind of cryptocurrency you wanna know. There are the centralized ones, such as XRP or stable coins, and the decentralized ones that are consisted by a peer-to-peer network. It's obvious that once you have a third party, the fees are much cheaper. That's the price of decentralization. 
Are they only low because of a low transaction rate and they could get as high as bitcoin if they reached that transaction level?
Not all cryptos have maximum block weight equal with 1MB, so no. You shouldn't compare it with Bitcoin. Altcoins offer you a nearly-zero fee, because of the transaction rate. Check monero for example. I haven't seen more than 500 transactions on the mempool. If the entire world started using monero, its median fee would increase. As said by Satoshi, it's an open market competition. Although, comparing to 2009, there aren't any nodes that will process your transactions for free. At least, not that I know any of. 
Is there no middle man?
On Bitcoin there's no middle man, like never. All the transaction fees go straight to the miners.