I think that the point being missed is for how long the status quo can be maintained. Rehnquist was part of the five-person "federalist" bloc on the Court (the same five who formed the Bush v. Gore majority). Although he did have his occasional lapses, such as the recent Tennessee v. Lane decision, he was indeed one of the Court's most reliable "federalists." O'Connor, though generally part of this bloc on states'-rights issues, was less reliable, so replacing her with an apparent true believer in "federalism" (Roberts) does solidify the bloc. It leaves Kennedy as the only somewhat questionable member, but that's mostly on cases involving social issues; he's reasonably reliable on the states'-rights stuff.
But apart from replacing O'Connor with a more ideologically sound justice, look at the "federalist" bloc Bush ends up with (I'm assuming a Roberts clone as the Rehnquist replacement, just for the sake of simplicity):
Scalia (69 years old)
Kennedy (69 years old)
Thomas (57 years old)
Roberts (50 years old)
Roberts II (50 years old)
Consider the tendency over the centuries for justices to serve until they are literally incapable of staying on the Court any longer (was it Brennan who responded to a question about retirement by saying he'd leave the Court feet first?). Consider also that as healthy 69-year-old white men with access to first-rate medical care, Scalia and Kennedy have a life expentancy of around 12 or 13 more years. I don't know about Kennedy, but I think Scalia will want to hang on as long as possible.
I wouldn't have said that a year ago. Scalia hasn't made much of a secret of the fact that he does not consider many, if any, of his colleagues to be his intellectual peer. But now that the colleague for whom he showed the greatest contempt--O'Connor--is gone, and he's got potentially three younger originalist/federalist [sic] justices who have been strongly influenced by his ideas, Scalia may well emerge as the leading figure in a bloc of justices that can reshape the Constitution over the next decade or so. Many of Scalia's frustrations (a lot of which can be laid at O'Connor's doorstep) can finally be eliminated. Indeed, depending on who Bush names as Chief, the Court of the next decade may thought of as the Scalia Court, not the Chief Justice X Court.
This is a much different situation than would have obtained had Kerry (or Gore) replaced O'Connor and the Chief. A lot depends on what happens with the other four seats, and with Scalia's and Kennedy's when the time comes. Without knowing who the presidents will be, who will control the Senate, what the other political considerations may be at the time, having a core of three solid "federalists" for the next quarter-century or more makes the probability pretty good that at any given moment in that period, the Court will be basically (or completely) conservative.
And if Bush gets to replace the 85-year-old John Paul Stevens with a 40-something true believer, I'm resigned to spending the rest of my legal career under a federal regime that resembles the 1920s: no deference to Congress, lots of progressive legislation getting the axe from the Court, growing popular unhappiness at the situation.