Until power meters can be had by all, avg speed is a good enough metric for most. I'm sure many like me follow particular routes and can see patterns of improvement over time. I, for example, can expect to maintain an avg of about 18mph over rolling terrain, and I found that to be true in the area(s) I ride near by house and yesterday during a century in an area I've never ridden before. So I think avg speed is reasonable when you look at the patterns, like routinely having 16mph over a loop to then 17 and then 18. Bottom line is, people want to see improvement and it's motivating to see numbers go up. If they want to use avg speed, let them and don't be a bunch of killjoys lol