Actually, I agree. When Claremont was writing, Byrne was a co-plotter. X-men lost a few steps when Byrne left. I don't think the other artists took as firm a hand in plotting as Byrne did.
In a lot of ways it mirrored what happened in Rom: Spaceknight. As good a writer as Mantlo was, Buscema, as artist, clearly participated in story development, and was often listed as such (the credits would read "Mantlo/Buscema, Writer/Storytellers/Artist", or something similar). And after Buscema left on Rom 52 or so, the story went way downhill.
I think some of this has to do with the different authoring techniques back then vs. today. Back in the 1970s and into the 1980s, in many cases (though I think not all), the process was as follows. The writer would write out a plot, as a bulleted list or a synopsis. It would say things like, "A spaceship hurtles to earth, and crashes in the West VA. mountains. The rocks split, and out comes ROM. Rom summons his energy analyzer and flies off looking for Wraiths." Etc. Then the plot would go to the penciler, who would translate those words into layouts and pictures. The penciler would decide things like how big to make the panels, how many panels to divide the scene into, where to put splash pages, and the like. Details and background features were put in by the art team. Then the writer would get the art pages back, and would write the script (Narration and Dialogue) to fit the art turned in by the penciler. By the way, this is why on some titles, you had plotters and scripters listed separately. Often artists would be listed as artists and plotters, and then there would be a scripter (Aragones did this a lot with Groo... he basically drew the whole thing out as story-boards, then sent them to Mark Evanier, who wrote the words to go with the pictures). Depending on the comfort level of the writer and artist, the writer might be more or less detailed about the plot.
George Perez once described how he and Wolfman did Titans. They would meet at a coffee shop and talk about the plot. Wolfman didn't write things down, but Perez liked to take notes. Then Perez would go home and from his notes, he would draw out the issue. He said that sometimes Wolfman would get the pages and call him up and ask, "Why is this happening on page 3?" Perez would answer, "It's in my notes. You talked about it." Wolfman wouldn't even remember. But he trusted Perez to both take good notes and make his own awesome tweaks to the plot, enough that Wolfman would just take whatever was drawn and put a script to it.
It may be that Claremont had such a relationship with Byrne (I don't think either of them has ever commented on it), and perhaps he didn't have that sort of relationship with the other artists. I've always pictured this as being the case, since the Claremont/Byrne run is so demonstrably better than everything else Claremont did by himself.
Cockrum was noted as not putting much in. I think his days at DC and earlier run at Marvel left him feeling burned but the companies, so just did the work.
Basically the Marvel Method either works or it doesn't. For some, it works well and other is doesn't and this can show.
There have been tons of interviews in the decades which said how it worked with Byrne and Claremont. Byrne and the editor did a lot of the plotting, but more importantly made it have a pace. Claremont was one of the first plot milkers in comics. He just dragged shit out, like a soap opera does, over dosed the angst, like a soap opera does, contrives the reasons for angst, like a soap opera does, etc. . Claremont was a dialoger and not much else. Byrne could plot and later showed on FF that he could also do the rest. But both worked with each other in a way, one not much different then others at Marvel. Claremont and Byrne spoke on dialog, tossed ideas around and brought out the best in each other. Like song writers... they ultimately had strong views and went from there. Just like Lee and Kirby.
No one pointed to Claremont's other work prior as great. None of his none X-Men stuff too. Claremont fans only bring up his X-Men work. Never his New Mutants, never the Iron Fist he made along with Byrne. Never his Marada The She-Wolf. Never his Excalibur. Never his Image work such as Huntsman. Never his Fantastic Four. Not even his Marvel Two-In-One work.
I read all that stuff.
I also have read nearly all the comics Byrne did prior, during and after X-Men. He did the major work on that title. It shows.
The Marvel Method, the editor at that time, the powers that be at Marvel at that time, the time itself all contributed to those X-Men comics. It all clicked. But folks can only see Claremont?
Claremont is a bad writer. But compared to a ton of other writers out there... he is one of the better ones. But there are a number better then he is.
As for his long run... Claremont had ideas, like some he had that life time job. For some it's the one comic strip. X-Men was his.
Hell, he probably can still fudge out ideas that are some what original compared to some that are happening.