Being there first means very little if you have a design that doesn't fit everyone's needs and isn't easily adapted to fit those needs.
The BSD kernel was there first. Does that mean Linus shouldn't have started the Linux kernel (even just for fun)? Sure, there were legal controversies around BSD, but that can be written off as "political reasons" the same way people are writing off licensing, ownership and policy issues with new stack components.
I don't understand why people take such a strong emotional angle on this. Extend, fork or rewrite, they're all different ways to develop software that fits your needs. Sometimes one approach is the right one, sometimes another is. None are morally wrong or always right.