Doctor Who - revived by fans, still running 12 years later.
JJTreks 1 & 2 - made by non fans - controversial in fandom
Beyond - Made by fans - most well received Kelvin film in fandom
Provided they are also talented, the idea that fans shouldn't work on their favourite franchise is utter hogwash.
This is opinion and cherry-picked data that you're trying to pass off as statements of fact.
First, there were both fans and non-fans working on every one of those projects.
Secondly, you pick a carefully phrased yet terribly undefined criterion - the supposed collective opinion of "fandom" - by which to measure the success of the productions you do decide to cite.
If you used a measure of, I dunno, how many people paid how much money for the opportunity to see those films - you'd get a result that pleased you less.
Uh, yeah - "money isn't the only measure of success" - but it does happen to be quantifiable. What you believe that "fandom" thinks is not.
Third, you just ignore inconvenient stuff that muddies the hard bright lines that you're trying to draw - such as the fact that one of the two or three most popular Trek movies ever, TWOK, was written, directed and produced by non-fans.
You can call that kind of thing an "articulated argument" if you like, but it's really a collection of unsupported assertions.
BillJ is not being unreasonable in his response.