I think you misunderstand people's skepticism. Very few people would not like more characters. I'd like the roster to have a hundred characters. But I'm not expecting that because there is no
proof of that many characters. Same with an additional 1-5 characters. I would love it, but I do not believe there is enough proof of more characters for me to believe it. If I were to believe in more characters, and this turns out to be the final CSS, then I (along with everyone else here who believes it) would be extremely disappointed. I would much rather be pleasantly surprised by unexpected characters than utterly disappointed.
With the All-Star lock, no one is saying that it
must have been taken at a different time from the other pictures, it simply
could have been, so it is poor evidence for more characters. It is unusual that Mr. Game & Watch, Duck Hunt, and R.O.B. do not appear on the bottom screen in their proper places in the leaked shot. Either the player had not unlocked them due to the picture being taken at a different time from the CSS leak, or the bottom screen does not update to include newly unlocked characters. If the former is true, then the All-Star leak means nothing, since it was taken at a different time than the CSS leak. If the latter is true, then it also provides no evidence either way because we have no indication of when the picture was taken. Either way, it's bad evidence for more characters.
As for clone central, the difference is that Dr. Mario, Dark Pit, and Lucina were upgraded from alt costumes, while Toon Link, Ganondorf, and Falco were developed from the start as their own characters. Furthermore, there is strong evidence that the 3DS roster operates by the sliding unlock mechanic from Brawl, where characters are always in the correct order, just with people missing in-between. Due to the flexible unlock order of characters, even if multiple players have the same number of characters unlocked, their CSS screens could look vastly different. Thus there is no consistent trigger that the devs could use as a point where the clones would disperse. So even if there are more characters, the clones would not jump up with their franchises, they would stay in the same basic order. Brawl's CSS never once violated the sliding rule, and SSB4 has yet to violate it either.
The fact that every single franchise is perfectly grouped together on the CSS and not split between rows is also very strong evidence that there are no more characters. With the sliding rule in place, a single missing character can scramble the entire CSS. Look at the incomplete Brawl roster below. Despite being only one character shy of a full CSS, Bowser is on the complete wrong side of the screen, Lucario got lumped with the spacies, Ness is now a Pokemon, and Sonic has joined his FE comrades. If the 3DS CSS was incomplete, you would expect some series to be broken up across subsequent rows in a similar manner (due to the series being arranged horizontally instead of vertically). I outlined in an earlier post that if we are correct about the sliding rule (which we very likely are), then the only characters that could still be hidden are Pokemon reps, Star Fox reps, characters from unrepresented series, and clones, and only in very specific combinations.
I'd like to reiterate, I would love more characters. I think Dixie Kong is a very deserving character, and I would be extremely happy to see Impa or Bandana Dee as well. I just don't believe it will happen. So I'll remain skeptical, and at worst I'll get exactly the roster I expect. Plus, there's a small chance I may yet be surprised.