This is a stance I strongly disagree with. I dont think anybody running Bitcoin projects left because sections of the forum were being spammed. They would have likely just been concerned with their own threads and their own users. Its when disrespectful forum members make it appear that their work isnt valued that they target their effort elsewhere.
Honestly, I haven't really seen this, and even if I have I don't remember it quite well. But since this is bitcointalk we are talking about, I see this being more than just a possibility. And I also dare say that most of them who were disrespectful didn't know jack about the particular project or even bitcoin. People weren't toxic back in the day. Well, I guess toxicity and spam is all you get when bitcoin becomes famous. 
But I still feel that valuable members, members who understood bitcoin,who were here in the early days, who weren't necessarily running bitcoin projects; left the forum because of the spam and its factor: Signature Campaigns.