For each halving, which is every 4 years, for the sustained profits, we expect either a doubling of total transaction fees or price of Bitcoin just to maintain the profit margins.
You just slightly estimated it. I don't know if it's on your realization, but it seems rather disappointing to me than (probably) to you. The price won't be that fluctuating forever; not that there's a proof for this, but over time, the supply will be steady and if the demand is too (global adoption), then the miners can only rely on the amount of the transactions.
Otherwise, the security will be slowly decreasing over time.
Coinbase transactions will always exist. They're the only required transaction in a Bitcoin block.
I guess he/she means in 2140. They'll reward 0.00000000 BTC, right? Just like OP_RETURNs.