Scientifically and formally I have to agree with needrain, the question of whether older cultivars are "fitter"/more vigorous/have more "vitality"/etc. is extremely difficult or impossible to answer because we cannot get a random or unbiased sample of the older cultivars while we can of the newer cultivars.
However, the theoretical answer is that the newer cultivars are less fit than the older cultivars and the newer cultivars are more fit than the older cultivars.
Before people started hybridizing daylilies there were different daylily species growing in different regions naturally. In natural conditions no one waters, fertilizes, weeds, divides, etc. The plants have to survive, grow, flower, produce seeds, with no help and while competing with other plants. People began growing some of the daylily species in gardens and crossed them with each other and began selecting them. In gardens daylily plants are watered, fertilized, weeded, divided or more simply grown under unnatural conditions. In the beginning the daylily hybrids will have been more fit for natural growing conditions and less fit for garden conditions. As generations of selection passed daylily cultivars will have become less fit for natural conditions and more fit for garden conditions. That is an unavoidable consequence of being selected in garden conditions.
If one grows daylilies in conditions that more closely resemble those in nature (for example, with strong competition from other plants or their roots) then cultivars that are less adapted to garden conditions and more adapted to natural conditions will on average do better or be fitter. That would be older cultivars.
On the other hand if one grows daylilies in conditions that are most different from those in nature (watered, fertilized, divided, weeded, without competition, deadheaded, etc.) then the cultivars that are most adapted to garden conditions will on average do better or be fitter. That would be the newer cultivars.
