I believe the bitcoin development is an unending process due to the fact that there is always room for improvement and advancement. Santoshi in 2008 set the ball rolling and subsequently development will follow. So I will boldly say it a job in progress.
Unending in a sense that updates in the future will not interact with the core features of bitcoin. As far as I know, the code behind the development of bitcoin is final, meaning, we don't have the capability to implement new features, nor make any changes to it. What we can just do is to develop the resources that we have such as hardware, and how we could efficiently connect bitcoin's feature to new technologies. Honestly, up to now, even segwit update didn't solve the issue of huge transaction fees completely. Mining as well becomes more and more complex and hard that hardware needs to adjust and improve.
code behind the bitcoin could be changed, but decentralized consensus is needed for that, there is a requirement for a broad consensus, or the changed code will be hard-forked from the main code
it is not likely to happen, but one should know that it is possible