The immutability of any given blockchain is reinforced by the total hashing power behind it. Proof of work (ergo a financial commitment) is a necessity to ensure the party verifying transactions is incentivized to do so honestly.
Currently (April 2018) mining profitability is relatively low and may soon become limited to those with really cheap electricity. This is before any scaling solutions have really come into full force with Bitcoin.
With the next block halving in 2020 (one of many more to come) and scaling solutions becoming mainstream will there still be enough incentive for miners to support the blockchain?