If the WPF given from no/low WM investment was lower, then it might encourage strength builds to put in more agility. This would lean them towards the balanced builds, which would mean less variety depending on how well the strength build users could do with lower WPF. But if strength-heavy builds (24/15, 27/12, etc) could cope with the lower WPF and related problems (weapon breakage, speed), and keep their high damage output and hitpoints capabilities, then a change to WPF might be okay.
Also, I think the comparison on the previous page against someone with 70 armour was a little off, I don't see too many people wearing 70 armour. 40-50 would be a more accurate number to go by.
Edit add-on:
OP, as for the difference in WPF at higher levels of WM for archers, why not put the extra WPF into a melee weapon instead? I always though anything over 4-5 WM was meant to by a hybrid. The points to fill from 160 to 172 WPF could fill out much more in a melee weapon proficiency. And if you do not want to melee, why not just take lower WM?