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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?
Oh, mining i.e. providing proof of work, isn't automatically validating transactions? (receiving transaction fees) – Josh – 2018-04-19T16:57:21.697
Miners need to be careful not to mine on a block that has invalid transactions, otherwise they won't get paid. So they'd be wise to do the validation. In practice most miners actually trust the pool server to have variates the transactions for them and don't do it themselves. Validating transactions required a full node. – Jannes – 2018-04-20T00:56:40.670
Ok, yes. I guess it brings up the next question of who is going to validate transactions once all the Bitcoin is mined and transaction fee's are virtually nothing. But a lot can change in 100 years ;) – Josh – 2018-04-20T09:30:52.513
That Q has been asked and answered many times on this site already. Short answer is: fees will have to be worth enough to pay for the mining. What "enough" means is still an open question. It does NOT have to be 12.5 BTC, especially if the price goes up a lot. But some equilibrium will have to form. That also means blocks will have to be always full, otherwise fees will go towards 0. That's another reason why "dumb" block size increases are pointless. Smart scaling like SegWit, LN and others on the roadmap, actually help set the stage for robust free market forces to develop. – Jannes – 2018-04-20T12:24:23.183