Just like any other network rules, the block reward could be modified through hard forks. If you convince everyone to run your client with the new block rewards, then you have effectively increased the supply limit. Anyone can do this, not limited to the creator. The whole point of Bitcoin is for anyone to be able to make changes and people can choose to or not to follow you.
Anyways, there is no economic incentives to do so. It would just devalue Bitcoin.
That's not entirely true. It really depends whether the original design's transition from subsidy to fees works as intended. The recent uptick in the fee portion of block reward is a good sign, but we still have very limited data. The premise that fees will replace subsidy without fundamentally threatening the network's security is still untested. It's theoretically possible that waning future block rewards could lead to hash rate drops that throw Bitcoin's byzantine fault tolerance in question. Severe enough drops in hash rate could allow older generations of hardware to come back online, opening up the possibility of massive block reorganizations -- and therefore utter unreliability as a value/monetary system.
If this were to happen, a perpetually low but predictable inflation rate -- which
guarantees some mining security incentive (fees cannot do that unless they are mandatory and sufficiently high) -- would look very attractive in comparison. That's your economic incentive.