Please correct again if I'm wrong.
I cannot speak for ArenMook but I think that when Unity was rebooting their GUI system, they looked at the most popular option at that time and it happened to be NGUI. It then makes sense to make the new Gui system so that it's something that most people are used to using. This way the migration process is smoother.
At that time, NGUI was very different then it is today. It was a different animal, much inferior to what it is today. The architecture is not nearly as mature. When ArenMook joined the Unity Gui team, many GUI systems were analyzed and the best features were taken into account in order to design the new GUI. One can also see that the brainstorming and contributions from the Unity GUI team has positively affected how NGUI has grown. One can say that NGUI today is a result of the contributions from Unity. So to say that ArenMook helped Unity to make NGUI obsolete is not quite right.
It's more like, Unity got some ideas from NGUI and in return, NGUI got lots of ideas from Unity.
Furthermore, if NGUI didn't contribute the initial ideas, it would not have made such a big difference because Unity's Gui team was determined to create an awesome GUI system anyways because they lost face with the series of Gui reboots. They really want to get this one right this time. It's a matter of personal pride with the unity founders.