Bitcoin code is technically easy to change, but if the changes are so big as to require a hard fork, many miners may not agree to change, and hence those code changes won't become effective.

On the other hand, new crypto currencies can build in the necessary modifications and then start their operations, or add the changes later when there are not so many miners yet. The more miners, the harder it is to get them all to agree to upgrade to the newest code. Miners are those who define the network operations and they decide what will fly and what will not. Bitcoin's being the first in this market is both advantage and disadvantage, the disadvantage is the code changes are too conservative and slow compared to other coins.

If people need such changes that a) Bitcoin developers might not agree to implement or b) miners will not agree to accept, then people could switch to another crypto currency. Actually this diversification has been going on for a while, top 5-10 cryptos from coinmarketcap if you're interested. Bitcoin is still the leader, because it still performs the basic function of relatively secure money transfer and store of value, and that's good enough for most people, for now.