Okay, but then all you're doing is pulling a "No True Scotsman."
As I noted - the tradition of house-ruling is very strong. I have, in decades of gaming, never played under any GM who ran a game
strictly as written. Every single one edited, expurgated, or otherwise ran a bit loose with things. Not because the game was unplayable without alteration, but just because they preferred ti that way.
So, insofar as "No true Scotsman"
fails to breathe, yes, I'm pulling that. However, I don't think that taking note of one of the most common things GMs do is somehow slighting or dodging the question.
In other words, we both agree that there are some rules systems that are, if unaltered, so bad that even a great DM can't rise above them -- he must change them instead.
Not quite: I can
imagine a system that bad - that's not the same as agreeing that such a game exists in print. I do not have personal experience with any that fit the description. I am leaving out HOL and FATAL, as I classify both as works of satire, rather than actual games.
If we assume such a game exists (it would not be hard to create one, honestly) I am unconvinced that anyone I'd call a "great" GM would ever choose to run with one it the first place.
If you must insist, "But what if he did?!? What then?!?" that's when I say they'd alter the system. But, I also think I have to pull the "too hypothetical" card. This feels like we are getting into the "Well, assume you *could* time travel..." kind of questions, so divorced from practical reality as to not be informative.
I don't personally know of any commonly available system that is so bad that I feel it absolutely could not be run straight and still be fun. I know of a bunch for which alteration would quickly make them more fun, but that's not the same thing.