Buy 'em while you can. B&M's will still be around when Internet sales eventually get banned, which will be beneficial for B&M's, and B&M's will probably stock more blends if they get more demand. Tobacconists are a business. If tobacco sells, product will be offered.
How many blends will disappear is anyone's guess, but I expect that the well established blending houses will continue as they have. Tobacco used to be much more expensive, with a far smaller number of choices. That changed. Now conditions are swinging back in that direction. But even if we go from 6,000+ blends down to 1,000, that will still be a lot more than were around when I started smoking a pipe.
If you stock up on what you like of the endangered species, you'll still be able to enjoy what you like.