3.5 was not a poor performer… I don’t think you know what words mean. 3.5 dominated the industry. Your very post is contradictory. 3.5 was revolutionary and the system that essentially props up the modern rpg industry. There was a reason game companies that exist today were formed to support 3e and then 3.5, it sold almost as well as 1e and continues, even now, to dominate conversations even after being OOP for almost 20 years. Green Ronin, Goodman Games, Fantasy Flight, Mongoose, Monte Cook, Alderac, some of the top companies today all started or kicked up their profiles as D20 publishers and especially proved themselves in the 3.5 arena. Even White Wolf survived for a few years on the back of D20/3.5 when nWoD didn’t exactly light the stage before they dissolved into PoD and then sizzled out. To call 3.5 a poor seller is like saying Return of the Jedi wasn’t a success because it’s box office was less than Empire.It's numbers were similar to AD&D which had been out of print 14-24 years at that point. That suggests a very very rapid collapse of the playerbase.
3.5 (a poor performer D&D wise) was still more popular and was putting up better numbers than AD&D while Pathfinder was a thing.