All current blockchains are fueled by artificial block rewards that pose long term systemic risk to the whole ecosystem. This is so, because eventually these rewards will be depleted and in case there are not enough transactions to incentivize the underlying nodes to continue operating at their current pace, this could trigger a potential collapse in node infrastructure/blockchain security followed by a potential collapse in value similar to a building collapsing under its own weight.
What would happen to the Bitcoin network if adoption declines (or remains steady at the current approximate 250,000 transactions per day) and eventually runs out of rewards? The economic incentive for miners would be significantly lower since there would be little actual economy behind the fees that these 250,000 transactions generate as compensation to the underlying infrastructure of nodes. The miners would have to either exponentially increase the network fees, so that the lower number of transactions could compensate for the eliminated rewards, or they would have to sell their mining equipment.
To give a rough illustration, if we assume an average Bitcoin transaction cost (average fee) being equivalent to e.g. $0.1, then the current 250,000 transactions (at the time of writing of this document) per day would equate to a total of $25,000 per day. In contrast, the current block reward is 12.5 BTC which is rewarded once in every 10 minutes. With current Bitcoin price being in the $6,500 range, that practically means every 10 minutes, a value equivalent of $81,250 is rewarded, translating into roughly $11.7M per day only from block rewards. The discrepancy between the value of the transactional economy and the value of the artificial block rewards is colossal. The actual economy related to the block rewards amount to 0.21%.
99.97% of the income of bitcoin miners comes from these artificial block rewards. Ironically, one can argue that the artificial rewards are somewhat similar to the quantitative easing that most governments apply in current centralized financial systems. The analogy comes from the fact that in both cases there is a non-market driven entity that generates artificial value in order to incentivize/boost the development of the underlying system. In centralized governments, that value comes from printed money originating from central banks which is then used to purchase securities from the financial markets. With cryptocurrencies, that value comes from the block rewards, that are generated by the blockchain itself.
Whether the technology for creating this value is centralized or decentralized, in both scenarios, eventually it will have to stop and market participants will have to adjust to it (and pay the corresponding price for this adjustment).
One can assume that a centralized economy is somewhat reliant on a theoretically (and rather flawed) infinite supply of this artificial value, whereas with the finite supply of Bitcoin for example, this artificial value is programmed to halve over certain periods of time and eventually is set to completely seize.
So far, block reward halving has been associated with spikes in Bitcoin value, which is linked to adjustment of market participants to this halving of the reward (and the assumed correlation between block rewards and mining yield), rather than linked to a fundamental increase in adoption. Without a doubt this has proven itself as an effective way to kickstart a blockchain, but unfortunately the actual adoption lags significantly out of proportion to this pace of growth.