Can you please explain why you want to keep the data flows after it got completed ? We need to evaluate this to see if we are missing any product feature. This is a default clean-up provided by Pega Marketing / CDH to delete it. Otherwise it could lead into other performance issues including the landing pages taking a lot of time to load.
If it is for temporary debug purposes and would be toggled off, there are options to enable it through debug configurations depending upon the release you are on. It's highly recommended to leave the default clean-up as is in PROD type of environments.