Here’s a graphic picture about the range of crop varieties available to farmers and growers in the United States. A range falling and falling between 1903 and 1983. Of course some of the now-extinct varieties will have been weak, but most of them probably would have been worth conserving. We don’t know what the future holds.
Since the 1980s, genetic modification (GM, genetic engineering) has gone mainstream. I don’t think that’s very good news for biodiversity.
Science on the Land 
I don’t think that the story is quite this bad. There seems to be a real resurgence in planting and saving seeds from heritage varieties.This company,, http://www.rareseeds.com/ is a great favorite of Organic home gardens and small market gardeners, but I don’t think that it’s considered a “commercial seed house.”
No doubt the bulk of commercial plantings are limited to a small number of varieties. For example, something like 95% of the US sugar beet crop is planted to a single roundup ready variety, but that doesn’t mean that 1000’s of small growers aren’t growing the varieties that their grandparents grew.
The risk of course is that we could easily see a massive crop failure of key crops, especially the corn and soy that provide most of our food. It would probably take years to produce enough seeds to replant all our acreage.
Yes. I’ve heard of the rare seeds company you mention, and others. Parallel to some of the rare-varieties suppliers over here. I like the Heritage Seed Library http://www.gardenorganic.org.uk/heritage which iirc doesn’t sell seeds, due to legal barriers. Gardeners borrow seeds from HSL, I think – it’s been many years since I read the detail so I might be wrong about it.
For livestock we have the Rare Breeds Survival Trust https://www.rbst.org.uk/
The rare breeds survival trusts and similar ideas are fascinating. Many of these breads, like the Vietnamese potbelly pig are small than usual and perfect for small homesteaders. The original “Jersey” dairy cow has become quite popular here.