You said that was not possible. Are you sure that old article is wrong?
No, the article isn't wrong, at least from the part you quoted. I'll give you a short answer that may be disliked, but it's true: If a Bitcoin node changes a consensus rule, it automatically stops being a Bitcoin node. By that, I mean that IF the majority of the nodes decided to switch from 21,000,000 to 42,000,000, they'd stop being the majority.
Note that this is very significant. No one can actually do something to Bitcoin, whether if it's the majority or the minority or all the nodes! There are some rules you have to follow. If you don't, you're not running Bitcoin!
Yes, the chances of switching that 21M cap aren't 0%, obviously. But, the article doesn't mention that if such thing ever happened, Bitcoin wouldn't change. Nodes would simply follow different rules, not the Bitcoin's ones. Thus, there wouldn't be any Bitcoin node running and so on, no Bitcoin.
Will consensus determine how much the transaction fees will be in the future?
The transaction fees don't have to do with the consensus. They're calculated by the amount of transactions in the mempool.