I see your point, but it's not going to work like that for me at all, and I won't be alone.
Of the six heroes coming at release, I'm only really interested in Cap. Say I level him up, finish the story arc, complete his Challenge card, and maybe it takes me 20 or 30 or even 40 hours, who knows? I'm having a blast! Now all I want to do is improve his armour/skills etc. and I'm even less interested in the other heroes. I'm having fun!
There's no way I'm going to play characters I have no interest in, for all that time again, just to "earn" some extra credits. So I keep playing Cap, buy him a costume or two, maybe I dabble a little on the side with the others, and I've got the 1000 credits saved. Hawkeye lands, here's my credits to "unlock" him, I'm keen, but JEEZUZ HE SUCKS ARSE!
No more credits, have to "pay" $10 for each and every hero from now on just to try them out, even if I never intend to buy another costume again. That scenario would have been impossible in Marvel Heroes, and I'll bet that's exactly why this design came down the pipe.