The behemoth-style rulebooks of rules may be, but I think Savage Worlds also falls into that category, and is doing very well in the hobby currently.
Savage worlds is very 3pp friendly, it has cultivated several complete games, and several that use it's relatively
thin core book.
Thin "core book" being the key.
That wouldn't have been possible, as Steve Jackson only got the license for TFT back a couple years ago.
Spirit not letter - they could have done TFT under a different name/serial numbers filed off.
But
total 20-20 hindsight. No way to know how the OGL would have worked out at the time.
You're discounting the large volume of setting books which are available for such systems. GURPS has plenty of books where the work has been done for you, including Traveler, Transhuman Space, Banestorm, and even a take on Star Trek. Their Dungeon Fantasy "GURPS stripped down to just fantasy" series also seems to be doing well for them. The previous edition of the game has even more material, everything from Conan to a complete line for WWII gaming, that's not difficult to adapt to the current edition.
Transhuman Space, Banestorm: IP that never took off - that is the risk one takes.
Either way it is moot. GURPS has an established reputation of being a complex ruleset.
SW and similar OSR games do not.
GURPS needs to have a game that shakes that reputation
if they want to be more than they currently are.
TFT has possibilities, but a retail of near 100usd is not likely to get people to try out the system on a whim.
But SJG may be just fine with what they currently are, without having to change a thing. So from their point of view I recognize that they might not see the ROI in making a riskier moves.
Powered by GURPS is a thing, which made many more self-contained books, which included a version of GURPS-lite instead of requiring the large core rulebooks.
But IMHO even GURPS lite, is not so lite.
It still has lots of fiddly things like variable skill and attribute costs, some combat calculations etc.
It could do with a ground up streamlining.
But that is just my opinion.
SJG knows their core audience, caters to it, and does relatively well in the scheme of things. They may see no need but to just keep doing what they are doing.
.