Anyone can crack the code. Just modify the block interval or the block rewards and you'll be able to increase the max supply. Problem is, due to your different block intervals (and as a result of the difficulty) and/or your different block rewards, you'll either reject the blocks generated by the network sooner or later. Effectively, you're forking Bitcoin. As a result, you're running a different Bitcoin with different rules that no one follow excepts you.
If you persuade everyone to follow your fork and replace the current Bitcoin, the effectively max cap would be raised. If no one wants to follow, nothing happens for everyone else.
it's related to the blockchain technology. if you modify one block, then this block won't be accepted by others and discarded at last. you're definitely not the first who thought about this, the designer had already figured the solution If you persuade everyone to follow your fork and replace the current Bitcoin, the effectively max cap would be raised. If no one wants to follow, nothing happens for everyone else.
so, it cannot be manipulated. buy it as much as you can