Interesting points..
>For example there are 50 years old apple trees in my neighbour's garden that are oh-my-god out of this world tasty.
Ha ha, I've eaten a few Indian fruits that are like that, both as a kid and later. One is jujube [1], called ber in Hindi. When living in Central India as a teenager, used to wander around in open, lightly forested areas near the town, which had these ber trees. We used to eat plenty of their fruit for free. Very tasty. Sweet. The still-slightly-unripe ones were the best. Overripe ones had a bit of a bad smell though. Yellowish-green in color when under-ripe, red when ripe, a bit smaller than, or the same size as a golf ball. I think they probably were high in sugar (fructose, maybe) since when sold in bazaars, flies were usually buzzing around them.
[1] https://en.wikipedia.org/wiki/Jujube
Another one was karonda [2]. Tangy, sour, astringent, may best describe its taste. Makes your mouth literally water - a lot. Punjabi people make a pickle from it. I had some of that pickle once in a Bangalore restaurant. Mind-blowing taste.
[2] https://en.wikipedia.org/wiki/Carissa_carandas