I don't think decay matters much, I voted for completely removing it. The point is that the system is not able to distinguish between JorDan and RiuT (really inactive) on the one side and e.g. Nicov and MbL (just didn't participate in the few tournaments that happened in the last time) on the other side. How does it matter that JorDan is ranked too high? He won't sign up anyway. Nicov in contrast will play HC3 qualifiers.Sure it's not without flaws, I'd say the decay isn't big enough yet, which is why JorDan is rated so high (same for RiuT). The reason why all the others aren't there yet is because they haven't had a deep run in a tournament where there was good opposition.
Because of these flaws I wouldn't advocate for a system where people are entirely decided based on the aoe-elo rating, but using it partially as rating is an improvement over purely using DE ELO I think.
Apart from that I agree. Mixing tournament elo in would be a good way to mitigate the problems with the voobly/DE ladders.