Honestly, I don't believe that changing the VA cast would hurt things that much. Even Funimation as VAs talented enough to imitate and sound just as good the previous voices. They did this before once with DBZ before making the dub their own thing. And let's be honest, if the Pokemon anime is failing, it's not because they changed the VAs or gave the dubbing rights to a different company. The new VAs have picked up just fine where the old ones left off, even if there was a rough start. The only reason why Pokemon is failing is because of the other reasons you said. Ash never aging, repetitive plots and cliches, lack of originality, etc. Sounds more like you've outgrown Pokemon, as have I. But it was never the change in VAs that caused it to go downhill or caused me to stop watching. And that anime is still on the air and probably still appeals to kids, so it must be doing something successful.
I think a change in VAs would be noticed, but if people are only watching it an enjoying it for the show itself and not caring about anything else, then they'll get use to it and will soon welcome the new cast. And I don't think it would effect sales either, because it's not the VA cast that determines if DVDs and merch will sell or not. It might be a factor for some customers, but it's not a major determining factor.