Some EX characters are cool (although Garuda is horrible - random polygon spikes coming out of his armor?), but it's not like there's a shortage of wanted characters that Capcom can implement without paying anyone else a dime.
Also, if they'd put in Skullomania or D.Dark, you'd have millions of people going "they put in THOSE guys over Guile/Blanka/Sagat/Sawada?".
You have people doing that
right now w/ Mika; not on this forum at least but in other spots. And not in the millions, but all the same you wouldn't have millions crying over Skullomania or D.Dark making the cut above others, either. All in all, that's a flimsy excuse to hitch the reasoning on imho.
Yes Capcom has a treasure trove of characters for SF but if that's what I think it is, they'd lull in characters from other franchises that have no reason for being in a mainline SF and put them in, just to MUGEN up the cast. IV already started to tip in that direction, ruining the close-knit feeling among its cast the previous entries maintained so well.
As much as I like some designs from Star Gladiator or Power Stone, those characters don't fit the Street Fighter philosophy, neither in terms of design or playstyle, and they'd stick out like a sore thumb to anyone even paying a little bit of attention to thematic consistency. It also requires taking into consideration that some of the EX designs are just outright better than a lot of what Capcom's squeezed out recently.
Let's face it, Juri and C.Viper were the only solid new entries from IV. In addition to them we got a boring French man, a stereotype Latino, a vomit-inducing insult rival to Ken, and a super-red big guy who has a hard-on for an oil fetish. If I forgot anyone then that speaks to how forgettable the other new IV designs truly were. EX even bosses over some of the III designs, which by and large were better than the newcomers in IV. Right now V is on point with Necalli, but we'll see if they can maintain that quality or get even better with the rest of the newcomers.
One thing is for sure, though; they'll have to push pretty hard to give us characters as iconic as some of the III, EX and Alpha cast; even Viper and Juri fall a bit short compared to most of them on several aspects.