An extension of C) is that this not only applies to open source. I was in a startup where the new employees decided to throw away working stable services to write their own version and they wasted a lot of time for little benefit. In the meantime the startup failed to deliver the necessary NEW functionality and we lost a lot of potential customers and sales people who were tired of waiting.

Working with other people's code is one of the most valuable skills. Be willing to live with existing code for a while and understand why it was implemented that way. Only then can you know what you should change.