The fact is that P&O is basically a 100% British cruise line. Therefore, the target market is much smaller than their multinational stablemates.
P&O has to operate within a constrained geographic market, with relatively few ships, if compared to, say, Princess.
So, they (Carnival) have taken the approach of launching two new mega ships, to attract new cruisers for the future, and let their more elderly client base continue to enjoy the older ships. In due course, both the older clientele and ships will disappear.
The limited market that P&O operates within, means that they cannot take the more gentle approach to which you refer.
However, i do hope that the next ship to be commissioned by P&O will be similar to the new Sun Princess, not another Iona.