Apple does the best job of anyone in the industry (imho) of building awareness and excitement over their events, developer or otherwise, but I think it is important to note that the WWDC keynotes (not the State of the Union), are also treated as proper consumer-facing product events, not just a developer keynote. As such, the audience is going to be much larger.
Back in the day, when Larry and Sergey were running things day to day, I/O definitely had more of that energy (they year Sergey jumped out of the plane and then walked on stage was bananas) and could pull Apple-like numbers, but that’s not what I/O is anymore.
40k concurrent for virtual conference that could be a series of blog posts and pre-recorded talks is actually pretty strong, IMHO.