The cumulative effect of all this is difficult to overstate.

Even if a PC has an ability score less than 20 by this level, it is guaranteed to be in a score that that character simply doesn't care about because its bonuses don't affect his or her specialty- for example, the sorceress mentioned above has a STR score below 20 (actually, I believe it's the one score she's never bothered to get a Manual for increasing- but I wouldn't be surprised if she does get one sometime within the next 8 levels since she, like the others, is practically swimming in resources right now). The scores that actually matter to the character- and I can guarantee that any player who pays attention will find that the save-boosting trio matters before reaching Epic levels in the first place- will be boosted. Spellcasters other than Divine ones may not increase WIS all that much, but they will do it for the slight boost to Will saves and the (more important) boost to the many WIS-based skills, particularly Spot and Listen (no PC likes to be caught by surprise). Melee tanks may not increase DEX much, but even they will appreciate having better Reflex saves and (more importantly) better initiative, plus access to new feats that help AC such as Dodge and others that help attacks such as Two-Weapon Fighting. And there isn't a character in the game who won't appreciate the all-important boost to hit points that come with a higher CON score- the Fortitude save bonus is really just icing on that cake for many.
The save-boosting ability scores of 40th-level PCs, in other words,
will be considerably higher than a laughably low 17. They may not be "prime requisites" for most, but they are important enough even without considering the boost afforded to saves that they will be earmarked for increases (not to mention, having higher scores put into them in the first place during the die-rolling or point-buying of character creation). A character who focuses all powers and resources on increasing the prime requisite, and neglects the rest, is not going to survive to 40th level in the first place.