Do you mean Milkweed and Laurie? Those are not really replacements.There are 2 bookstores downtown that are small businesses. Use those.
Laurie is North Loop, not downtown, and has a quite odd selection of old collectible books. It's fun, but I can't go buy the new Adichie novel, or Black Panther comic.
Milkweed is a better replacement, but is also not downtown (it's further from the IDS than the MIA is), is quite small (700 sq ft vs B&N's 25,000 sq ft), and focused on independent publishers, themselves foremost. Almost half their stock is poetry. Which is great! I love that store. But it's not a replacement.
I will miss having Barnes & Nobel downtown.