What you said is true to an extent...
Out of all the villians you listed (Vulture, Scorpion, Rhino, Black Cat(not technical villain, but ya know) Kraven, Morbius).. only Black Cat, Kraven, and Morbious have compelling emotional story arcs IMO. Scorpion and the Rhino are just goons to test Spidey's powers, and I can't see the Vulture's arc being compelling enough to sustain a Spidey film, especially one who's target audience are teenagers (C'mon.. who really wants to watch Tobey beat up an old geezer?)
I actually think if Raimi didn't choose Sandman, then the next obvious choice would have been Kraven because the character is very complex and three-dimensional.
If they did the Morbious arc, I think the only way they can tie it to Peter Parker/Spider-Man emotionally is if they explored the Mutation story arc where Peter starts turning into an actual six-armed man-spider.. which is personally in my opinion not a good direction for the franchise to take.
Black Cat was considered to have a significant role in the first print of the Spider-Man 1 script, but was cut due to the origin story of the Green Goblin needing to be done justice. They cut her out of SP2 for the same reasons with regards to Doc Ock. And, I am guessing, they cut her out of SP3 since they are already introducing another love interest for Peter Parker, Gwen Stacey. Avi has gone on record to say that they do have a Black Cat story arc already scripted, but just couldn't fit it into the first two films.