Foot traffic is the least important part about what E3 does. Its about the general media coverage and the boom of online viewership. An event like this is covered by the media in a way that gets the attention of super-casuals and its even sometimes talked about on local news stations or cable news channels, etc. I'm not saying Sony can't replicate viewership in other ways, such as dropping its own presentations and info videos, but E3 serves you up an audience on a platter. And it gets the attention of people outside of gaming circles.
Here is a quote from a response article at GameIndustry.biz..... and keep in mind these are stats from 2018-2019 shows, which would likely pale in comparison to the attention brought to a potential E3 2020 with new consoles launching.
"The numbers speak for themselves. According to Fancensus, over 70 million people watched E3-related Cyberpunk 2077 videos on YouTube, 37 million watched Marvel's Avengers footage and 24 million enjoyed Watch Dogs Legion YouTube coverage. If you compare that to Gamescom-tagged coverage, probably E3's biggest rival, the most viewed game from that show was Marvel's Avengers, which generated just under 9.2 million views. And, in the past, PlayStation has dominated E3. In 2018, Sony's products attracted just shy of 80 million views on YouTube (again Fancensus data) during E3, just under 10 million more than Nintendo and almost 55 million more than Microsoft." (https://www.gamesindustry.biz/articles/2020-01-13-is-playstation-right-to-leave-e3)
It's important to this conversation because it's a big reason why Sony isn't attending. As the article notes, Sony is one of the companies that wants E3 to become more of a consumer show. The ESA continues to drag its feet on the issue so Sony walked. They voiced their displeasure for the direction E3 has been taken back in 2017. So they're looking at it from the perspective of those who're attending. I don't think they're concerned about generating hype for their shows or games for those that can't attend. The hype leading up to the 2013 PS Meeting shows they have no problem doing that. And with them checking out of E3, it immediately puts a lot more attention on any other show that they do.