Not really true though is it.
ISO performance increases with each generation.
Where you might need an f2.8 with the 'old' body an f4 might be fine for the 'new' one. A good f4 lens costs a lot less than a good f2.8 lens (lighter too). Or you can 'afford' to stop down your less-good lenses to sharpen them up.
Dynamic range increases with each generation. A better lens doesn't help here.
If a generation old body is much cheaper than the new one then maybe you can buy one better lens. Maybe that's enough to offset (or more than offset) the advantages of the new body, but all your other lenses will still be at a relative disadvantage.
Bodies are cheap and you only need one. Lens are expensive and you need lots.
Skimping on the body makes little sense to me unless you just want one specialist lens.