So, one thing that’s I need to get off my chest is, he could plant pretty much any decent cultivar of plum with the right chill requirements and they’d be just as well adapted as the ones from his seed grown population. Aside from some summer drought, which arguably is good for most stone fruit anyway, it sounds like his island location is a pretty nice place to grow plums. There isn’t much benefit to having a lot of adaptability in a cultivated plum population there, because there’s not much to adapt to. And, more importantly, a population that’s inbred long enough to become a landrace is going to be pretty darn genetically uniform. So if he were to breed his plums and plant their seed until he got a stable landrace, because there was so little disease, environmental or other negative pressure on his population, that resulting landrace would not have much genetic “room” for adapting to a new negative pressure were it to show up.
On the flip side, I live in the coastal plains in the South, and here virtually all Euro and Japanese plums will die after a few years unless drenched in chemicals. The only trees that survive well enough that you could maintain a population of are the native plums and their hybrids, and even a lot of the hybrids don’t do well. But, here’s the thing, fruit quality, compared to the plums that have been domesticated for a thousand years, sucks. So if I were to just OP and plant seeds for a few generations, I’d end up with plums that will survive here, sure, but they wouldn’t be worth growing because the fruit aren’t really worth eating. Here, the disease and environmental pressure on the population is too high. Anything that survives being bred with itself enough to create a landrace here would be all but guaranteed to have lost any genes for fruit quality along the way.
It is already extremely difficult just to breed hybrid plums with the combination of disease resistance and acceptable fruit quality to be worth growing in the South (without tons of spraying), so why make life so much harder by adding an even steeper requirement?
Again, it’s the process of pushing the population into enough uniformity that it becomes a landrace that’s causing the loss of genetic variation. So why push for a landrace, when we have techniques that are thousands of years old that work way better?
Landraces make sense for annuals, sure, but for fruit trees is seems a vast waste of resources. Better to curate populations that have a lot of wide crosses in their parentage as well as careful backcrossing, that way you can actually pull off those magic and rare combinations of traits that make a fruit tree worth growing. Which, again, is what professional and most competent hobbyist breeders do. And when they do get that one-in-a-thousand plant with all the right traits (a plant that would have been one-in-a-million in a natural OP population), they save it, clone it, and spread it around for all to benefit from.