1. It's not meant to provide parity between PC and monster ability scores, but rather less difference between a PC's most important score (where he can no longer apply inherent bonuses on top of advancement increases) and secondary abilities (where now he can apply more than 5 from wishes and tomes/manuals).
2. Melee is nowhere near impossible. You can still reach 23 before bonuses, meaning 29 (+9 bonus) with bonuses (even restricting things to items), so even a fighter who picks entirely useless feats (which is essentially an NPC class) can get 20+9+5(weapon enhancement)=+34 bonus. That's quite enough to hit anything (except a defensively optimized PC-type) with a very decent chance.
3. PC save DCs will actually tend to improve as a result of this change, since the "inherent bonus" slot will boost their secondary abilities rather than primary abilities, and saves are usually based off secondary abilities. This was actually part of the original goal of the change.
Remember, in the old system you were also capped at 18+1/4 HD, simply because there's no way to start with more than 18 or increase the base score more than once every 4 levels. So the only way it weakens PCs is by removing inherent bonuses with the replacement not being usable on the key ability score.