It may be worth noting that, while the Concorde had smaller seats than the first class seats of that era, those first class seats were still nothing like modern lie-flat business.
The point of lie-flat seats is to let you doze off on long trips. The point of Concorde is that even trans-Atlantic trips weren't log enough for that to matter. You don't get lie-flat seats in first or business class on many US domestic flights either, for the same reasons.
That’s starting to change though. United is reconfiguring a lot of their aircraft for Polaris seating even domestically. The seats are more comfortable though I agree it doesn’t make as big a difference on even long daytime flights.