The actual theoretical output for the Forge is definitely questionable. But that is ALWAYS the case when it comes to theoretical frameworks for abstract things. How many hundreds of years did people in human history apple the theoretical frameworks of Aristotle and Plato despite evidence to the contrary?
However, discussion surrounding and in the Forge did generate a lot of innovations to RPGs. Some of the most influential games have come out of this Think Tank. These may not have been the games that the most players were playing, but they were often games that other designers were taking note of. Likewise, those who often felt the recipients of the Forge's scorn (e.g., certain types of D&D players) were unquestionably influenced by the Forge. IMHO, part of the success of the Old School Renaissance movement came from taking the criticism of the Forge seriously and responding in kind to the validity (or lack thereof) of those criticisms. It spurred the OSR to find the language to talk about its game philosophy and the intentionality of its game design.
I don't exactly think it's some sort of coincidence that there is actually a lot of design, brain-power, and play overlap between the indie story games and the indie OSR movement, a point that probably people in the latter category would likely not want to admit. This is one reason why I am attracted to a number of OSR and OSR-inspired games as well as the story/character-focused indie games (e.g., Fate, Dungeon World/PbtA, Blades in the Dark, etc.). These games often have much a clearer design focus regarding their intended play experience than many mainstream games (e.g., 5e D&D).
If the Forge succeeded at nothing else, it pushed us to think about how we talk about TTRPG design and play.
