Absolutely - and both are vital in ensuring effective and accurate communication
I hear what you're saying and agree, but it doesn't dispute the fact that only by using 'Granddad' is the 'd', to indicate the necessary 'Grand', actually pronounced.
Lol - love it!With the gradual homogenisation of spoken language either pronunciation could become the norm, and so that spelling might become the predominant one.
I wouldn't lay bets on it going either way, 'cause you might well loose your money...
Marco.