DannyA - you're comparing 3e core to 4e core. Sure, you can excise all you like, but, in 4e, the presumption is that rules are there to be used.
I couldn't care less about the economic impacts, I worry about the in game impacts. Group 1 uses the Big Book of Mundane Items and gets a couple of plusses to just about every skill plus a number of mechanical effects. Group 2 doesn't use the book, or excises those rules from the core.
Who do you design for?
Sure, if it's only my group, then great, but, it's not just my group. You mentioned psionics, and if a designer presumes psionics, that's a mistake. Ok, but, psionics are not part of 3e core rules. You're absolutely right they shouldn't be presumed.
But, can I presume that the party will have access to Fly spells? After all, that's core, and certainly not a terribly difficult thing to get in 3e. Should I presume that a 7th level party has access to Flying magic or not? And, why or why not?
And this becomes the problem. Every time you add more of this stuff TO CORE, the designers have to presume that it's in play. And, like it or not, in 4e, anything that appears in a PHB, DMG or MM is considered core.
Now, this 3pp PDF? Fantastic. Love it. Great stuff. No designer is ever going to presume that it's in use. Anyone who wants to use it can do the work of adding it into the game. More power to them. OTOH, I don't have to remove it from anything. Everyone's happy.