Wasn't their retirement a political, rather than an operational, decision?
I think Dennis Healey wielded the big axe in 1977 having decided that as we were no longer a significant presence east of Suez we didn't need such a large transport fleet.
Thus the Argosy, Belfast and Britannia fleets all got the chop and I believe even most Andovers departed under this plan too. This was not about selling the family silver to raise money but to save on running costs. All were sold off pretty cheaply (£50,000 for a Belfast, I gather).
So the question of whether these types were good or bad didn't enter into it - our political masters decreed they had to go and they did.
I do recall reading that the Argosy had its share of shortcomings from an aviator's point of view so it would be interesting if some ex-pilots would let us hear their experiences of the Whistling Wheelbarrow.