I got to see a side by side comparison at the last SPA match I attended. A guy had a Dirty Birdy ARF converted to electric among 3 other Dirty Birdy ARFs that were glow powered. The electric DB had better acceleration than any of the 2 stroke planes (not that that is a big deal in SPA, but it did have it) and was at least as fast. It didn't look any more heavily loaded than the glow planes; if anything it looked a bit more nimble. It didn't suffer a bit in vertical performance, but to be fair the 2 stroke DB's with the Jett mufflers didn't either. The plane was very competitive. Yes, setup and pilot skill are much bigger factors than the power system, but the point is that the electric pilot suffered no disadvantages. And that's with just an ARF conversion. If a designer were to take the DB shape and lighten it up in those areas that were originally built a little tougher to handle the glow vibration, another 3-4 ounces could probably come off of the plane. And as batteries continue to get lighter, the weight advantage would continue to increase from where it is now.
I don't mean to start a versus conversation by saying any of this. I simply mean to show that for the first time since RC planes were invented, there is an alternative to glow power for these midsize models that at least matches its performance and sometimes exceeds it. That's why glow is declining- a cleaner and more convenient alternative exists that we would have used all along had we had it.