Remember that all cryptocurrencies run under a consensus agreed between all its participants. This consensus is always subject to change: the Bitcoin network could decide to allow for 10 million more BTC at any moment. They just choose not to do it.
Ethereum builtin limits work as an incentive to force timely decisions. The initial 100M limit, and the difficulty bombs (which cause "ice ages") ensured that the initial versions of Ethereum wouldn't be able to run forever: mining rewards would stop, or mining would be too difficult to be profitable. This impending doom ensures the protocol needs to be updated, and once that conversation regarding the new consensus is active, it is easier to bring in new changes and improvements too.
One final point: whoever bought ETH at any point up to mid 2017 has seen massive returns. It could be argued that this is precisely because of the changes Ethereum has undergone over time, and that keeping close to the initial design would have been worse.
Ethereum builtin limits work as an incentive to force timely decisions. The initial 100M limit, and the difficulty bombs (which cause "ice ages") ensured that the initial versions of Ethereum wouldn't be able to run forever: mining rewards would stop, or mining would be too difficult to be profitable. This impending doom ensures the protocol needs to be updated, and once that conversation regarding the new consensus is active, it is easier to bring in new changes and improvements too.
One final point: whoever bought ETH at any point up to mid 2017 has seen massive returns. It could be argued that this is precisely because of the changes Ethereum has undergone over time, and that keeping close to the initial design would have been worse.