To be honest, I do remember being annoyed with BW's reset Pokedex, but only because I couldn't catch my favorite at the time without transferring from DPPt (It's Lucario, for anyone curious). I eventually came around and started liking the games (Braviary is in my Top 10), but I feel like if BW came out today, the reaction would be the same as SwSh. But that's just the world we live in, eh? XD
I also agree that this is better for SwSh, because it means they can flesh out the game as a whole, not just add new areas. And if I had to hazard a guess as to playtime, I'll assume it'll be a fairly large amount for Isle of Armor, since it revolves around raising Kubfu at least until it evolves and training at Mustard's dojo. The levels for wild Pokemon will probably higher as well, since most folks will have full teams of Level 100s by the time it comes out. We don't know enough about what goes on in the Crown Tundra storyline-wise, so I can't make a better guess than roughly as much as Isle of Armor.