I think a great deal depends upon where you are located.
Roses cheered as disease-free in some areas are disasters in others. Different environments. Different diseases.

Also, I don't think the old varieties are becoming senescent.
I've had a few mildewy Austins, for that matter. :-) And some rusty ones, too.

I DO believe, however, that some budwood (of roses both old and new)
may produce clones of lesser value. Perhaps it's a case of very minor sports -- generally speaking identical to the mother plant, but deficient in one way or another.

Jeri in Coastal Southern California

My Jude is definitely of a light if variable yellow color, occasionally perhaps with minor shades of light orange/apricot, but I wouldn't call this an apricot rose. It's yellow.

My Austins are doing quite well here in the UK without spray, usually better than my HTs and even OGRs, but it really depends on the variety and climate/position, so any generalization has to be taken with several grains of salt.

And as Jeri said, even within any given variety (and not just in roses), some plants are stronger than others. I may be wrong, but I don't think that it's necessarily a matter of minimal genetic variability (although this could conceivably also be the case, given the complexity of genetics), but rather how well they have been grafted (if grafted) and cultivated from the beginning. Budwood or cuttings, even if taken from the same mother plant, don't always take in the same way and some remain weaker than others. Some roses may also be more prone to decline with age than others, especially if not in a perfect position/care. So replacing a declining rose with a new one of the same variety can make a difference without the presence of any genetic difference.

Generally, David Austin and many other breeders are now increasingly breeding for disease resistance and performance, not just looks. But even this does not always work, as you can see with Austin's fairly new rose Wisley - a most beautiful rose and great performer which is unfortunately extremely prone to BS and has now been replaced by DA with another variety (Wisley 20008). But generally speaking, the newer Austins are more disease resistant than (some of) the older ones, and most of them are better performers. Within other classes, HTs/floris/polys/OGRs I found that it really depends on the variety, although many HTs can be quite lousy in terms of disease resistance. At least in the wetter climates of the southeastern US, UK and northern Germany.

Andrea