I wonder how much outfits like Send Cut Send are eating into 80/20's market share. I know I've used laser cut sheet stock for a few projects that probably would have used aluminium extrusions in the past.
I'd love to get some links on those cheaper extrusion websites you're talking about.
In most cases you need a sheet metal press to form a scaffold out of _just_ sheet metal.
For anything structural (ie: when thickness matters), the price of custom-cut sheet stock is not that competitive anymore.
The price of machine-cut metal sheet has dropped quite a bit the last years though. I think it's filling a new niche, not really replacing the space of modular extrusions.