The advantage of SBCs aren't just the price, it's the uniformity. Before SBCs made it on the scene, the default response when something doesn't work was either:

"Well, it works on my 12-year-old Thinkpad"

Or

"Your system sucks, just buy better hardware"

Now there's a consistent system to build compatibility for and benchmark against that can help the most people, allowing devs to spend time on things other than hardware, and allowing a "how-to" community to flourish, bringing new users into the fold.