Honestly, this is just how research and writing works. Is it better to have a comprehensive, perfect book to start with? Sure, but frequently the act of publication is what draws extra eyes to a subject and makes it clear where the mistakes and holes are that need further work. I won't presume to say that we wouldn't have the K98k books without BBotW, but if nothing else I think it's fair to say that the existence of the latter focused a lot of minds and got people talking about the observable discrepancies. I remember when the guidance for 98k resources was to buy BBotW but also keep a notebook by it so you could take notes on errors and omissions as inevitably discussed on message boards.
Don't get me wrong, I wouldn't recommend anyone buy BBotW today, but it's hard to crap on it too hard given the foundation that it laid and the conversations that it started. In a lot of ways that's the lens I view both Hitlers Garands and Rough Forged through. In a perfect world I'd like to see a robust community over on a G/K43 focused board really pull them apart and generate some good, published writing to correct them. (Maybe that's already happening, I'm not dialed in on the SVT40 community, for example).
As for sole authorship, it has its plusses and minuses. I don't know Weaver, never talked to him, and I have no idea what his motivations were for doing it this way. Maybe he welcomes input from fellow collectors, maybe he rudely rebuffs it. I've seen both in this hobby. What I can say, however, is that for all its obvious advantages collaborative work has its own challenges. Doubly so when the people collaborating are doing it as a hobby or a side-job. Even in academia, where everyone has "researcher" as one of their main job duties (regardless of however much teaching, administrative duties, advising, etc. can steal most of your time), collaborative projects are challenging at best. People have different priorities. Major stuff happens in their lives (births, deaths, divorces), families need tending to, etc. Frankly I'm extremely impressed that the 98k books came out at all, to say nothing of the high degree of quality. With these sorts of collaborative projects invovling individuals who have day jobs that's the exception rather than the rule.