Ehh, I'd argue the casual fan argument isn't as clear-cut as you make it seem - there's a significant portion of "casual fans" that watch games of the team that they like, as long as they're not horrible. Case in point, the only other Spurs fan I know IRL is what I'd call a "casual": doesn't follow the team too deeply, watches about 1 in 5 games, but knows about the players and what they do. He falls out of both of your definitions, yet is still a casual Spurs fan.
Of course the Spurs could get lucky and "only" have to tank out a year or two before getting that cornerstone. Again, this isn't an argument against tanking. But you just can't tell me for sure that that'll happen, and in the same vein, the Kings and Wolves of the league once too thought they could turn it around easily. Turns out it's much, much harder than pro-tanking folks make it look - , just look at Boston right now to see what can go wrong with a tank job (someone's bound to quote me on this to argue Boston didn't truly tank, but w/e) even when you draft the correct players and they turn into stars/superstars. No guarantees.
And it again brings me to my last point - yes, I really believe this mediocre team is better to watch than a team where we trade all 4 of our vets for scraps. Again, "hope" is of zero interest to the average fan - nobody wants to see the evident growing pains of a young-players-only team losing 60+ games a year. People wanna tune in once that phase is over with. You, as a hardcore fan, would probably be enthralled (don't get me wrong, to a certain extent I would be too; but this isn't about fans like us) to see every young player given free reign to make mistakes and learn from them. But it almost unarguably makes for a ty product, and I can guarantee you that most people wouldn't be too interested in the young guns getting blown out by 20+ every night.