Not all promo stuff ends up as trash right away. Some of it people actually keep and use, like totes or water bottles. Cons are getting better about giving out less junk and more useful things. The real problem is when stuff is so cheap it breaks fast, but that's changing too. I've seen folks reuse con swag for years, so it's not all waste.
What about all the promo junk that gets handed out and tossed? Those plastic flyers and cheap freebies create a MOUNTAIN of trash before the con even really starts.