Years ago I read that in the early years of the US there existed a huge number of different varieties of apples, as compared to the skimpy selection now available in supermarkets. (just googled & found this)
There are seed banks and culture tissue banks around the world, plus any number of botanical gardens engaged in preservation. I found this article on one preservation attempt most interesting. Click the quoted text for the full story.
Obviously, all heirloom plants should be saved (isn't there a world seed bank somewhere in Scandinavia?), but clearly we don't want all of them to come back right now. They are there in case of an emergency (or a huge change in taste). I have bought things like black tomatoes or purple potatoes at a premium and frankly they were absolutely not worth it in terms of taste. And I am not a fan of parsnips. I mean white carrots, really? And they don't even taste like carrots. They don't taste like anything.
But some day, any or all of these things might become essential. After all, around 1870 about 90% of all of the vineyards of France (and the rest of Europe) were wiped out by phylloxera. Crossing the European grapes with American grapes that were resistant to phylloxera created hybrids that saved the industry. Things like this might become extremely important in the future, because you never know what sort of food catastrophe might hit the world. Imagine rice or wheat being wiped out. We all know what happened in Ireland with potatoes.
What happened in Ireland with the potatoes happened because every single potato plant in the whole country was cloned from the same parent -- in effect they were all the same potato.
As far as the black tomatoes and purple potatoes you've tried and didn't like, I'm not sure that planting trendy vegetables under delicately perfect conditions would produce the same flavors those items would have in their natural, probably very limited habitats. The Vidalia onion, for instance, when grown outside its legally circumscribed area, will generally taste like a regular yellow onion.
I used to subscribe to the most wonderful newsletter when I lived in northern coastal North Carolina. It was put out by the county agent and heavily used by farm wives. People swapped, offered, and looked for seeds and plants, overwhelmingly the old varieties known in that area. These newsletters existed all over the country, but I don't know if they've survived the digital age. I learned of them from the lovely book, Gardening for Love.
As for parsnips, you're just flat wrong. I was in my 20s when I finally had my first parsnip, and I was absolutely delighted with it. I would buy them all the time if they had them where I live.
I planted two black cherry tomato plants this year. The lady selling them told me that everyone who bought them last year came back for more.
Not long ago, when digging through the books we moved, my husband found a French gardening book from 1865 called "Gardening for Small Gardens". In it, the author talks of fruit trees. The list of apple trees took up a whole page, but I only recognized the names of two: reine des reinettes, which still exist, and Cox's Pippin, which I have seen in English recipes. The others were completely unknown to me. I get the impression that the different varieties I see at the market are more recent rather than old ones that are being brought back.
Plant sellers have lots of different tomato varieties now too: for years they sold only two or three kinds.