Boron wasn't just lighter, it was a lot more sensitive than the graphite of the day. Unfortunately, it was also more brittle, and had this nasty habit of shattering when put under a good load, particularly in cold weather. You're right, more R&D would have figured it out, but Fenwick decided it just wasn't worth the investment given Boron's substantially higher cost and much, much lower sales volumes.
I got a couple of Fenwick Boron-X rods shortly after they first came out. First one blew up in my hands spectacularly while I was fighting a fish, leaving me holding a cork handle and a stump that didn't even reach to the first guide. Second one split when I set the hook on a rock. Fenwick did replace them both under warranty, and and I subsequently swapped them for graphites.