Dio vocals do make songs feel longer to me. I think anyone would have that perception when the singing voice is unpleasant to the listener's ear. (I would also argue that many Dio songs are lyrically and musically repetitive.)
For what it's worth, I like keyboards. In fact, the few Dio songs I don't mind have keyboards
A long song can be a masterpiece. Too often it's self-indulgence instead.
Also, I've been listening to music for a few decades. I'm not some loudmouth teenager.