I don't think it's offensive at all to speculate on why one believes an author's skills are deteriorating. Most authors' skills decline as they age or if their mental abilities become impaired due to alcoholism of substance abuse (just read the later novels of William Faulkner for evidence of serious literary decline for both reasons). It's nearly impossible to keep up a high level of ability, particularly if you've written as many books as Pterry has.
In any case, Pterry has been very open about discussing how Alzheimer's has affected him and how it has affected his writing. He can no longer "type" stories and instead must dictate them, which is an entirely different way of creating a narrative and one that can often lead to narrative inconsistencies and excesses, since he can no longer physically 'edit' what he's written. Like many others, I believe that while Alzheimer's has not impaired his ability to write books, or to even create compelling stories, I do believe that what I and many others see as a drop in the level of quality in Unseen Academicals and Snuff at least may be attributed, in part, to his condition. I believe that his recent YA adult books (ISWM and Nation) were 'truer to form' because they probably were subjected to a strong YA editorial presence that may have toned down some of the excesses. I don't believe that anyone is truly editing his adult books with a firm hand, which would explain their many narrative problems.
Again, the fact that he can still produce books at all is something to be cheered and when a new book is published, it should still be a cause for celebration. But that doesn't mean even dedicated readers have to take a fanboy attitude toward everything and refrain from criticism and speculation on why they see this apparent deterioration occurring. If you disagree with it, fine. Is that what literary discussions are all about?
As a very public figure, Pterry is subject to any kind of speculation about the reasons why they feel his writing has suffered that readers can offer. If people believe he's writing BETTER because he has Alzheimer's, that's a perfectly legitimate point of view as well.