It's always been easier to sell something with even nominal rear seats, even if you wouldn't want to sit there.
Look at the 911 or 944 for example.
Even the Miata has gone upmarket a bit, leaving nothing at the bottom.
Gotta think that almost all new cars being financed has a lot to do with this. Adding features adds profits and it's easier to sell when "it's only a few more dollars a month". Seen something similar in motorcycles, with the profusion of sportbikes & cruisers folks asked for a new "standard" bike that was less expensive as well. Each time I saw one introduced, sales were poor & they didn't last long.
Mustang might have made it's "rep" with the Shelby's & the GT, but it started as an inexpensive sporty car anyone could own in place of an economy sedan. The new V6 models, with decent performance & economy at a more reasonable price might help sales again. There's more competition now for that market - IMHO the Focus had a similar start to the original Mustang & now even that market is split with the Fiesta filling the bottom end.
I'm tempted by the Mustang myself, it's a car I always liked and haven't owned yet!
It might even replace the P/U, I can always use the excuse that it'll pull a trailer when I
need to haul junk around....