Levaing your old users is definitely not a good idea. Not only in software development but also in real life this is applicable.
Imagine you have a growing product. You should not only cater to new users but you should retain your old users as well.
You can’t just simply leave people who helped you grow behind while building a new network. Every individual who contributed for the growth of individual / your software application is important.
It’s like remembering where you came from, and also having a vision of where this is going. Decisions that you make as an individual, decide where you go.