I think what Clay has done for the hobby has been immeasureable. The time and devotion cannot be questioned. Now that he has taken his bat and ball and gone home, someone else will fill his shoes. The Wiki is the start, and I hope it develops for newbies who are looking to get into the hobby.
The reality is that Clay's work was the combination of his hard work and that also consisted of contributions from other Pinball people. I can see that contributors might not be pleased that their contribution now has to be paid for. The exposure of the Marvin's guides and the wider audience that were attracted to it will simply follow the next leader. Forums, Wikis and the collector communities that have grown over the last 10 years won't disappear as quickly as the Guides.
If you need to know something, google is your friend and so is AP