You probably could take out a demigod like Finder simply by killing all his followers as he doesn't actually have that many at all. However, having so few followers he takes a very close interest in them and hence would notice if you killed even a few. Even if you could deal with Finder, then he has a sledload of greater power allies like Chauntea, Oghma etc.
The other weaker demigods are mainly ascended mortals closely allied with good powers. They wouldn't have survived homicidial gods like Shar and Talos (both fond of subsuming lesser powers) or Cyric without powerful allies.
Even a almost universilly disliked demigods like Velsharoon still has Mystra as a protector.
All this talk of portfolio stealing is redundant really, seeing as Ao has to approve a god and his portfolio upon his/her/it's ascension. If the ascended mortal's alignment and general worldview doesn't fit both their portfolio and Ao's Balance, then they'll simply be hurled back to earth (if they're lucky) or simply obliterated completely.
And Shar is a selfish bitch, she wouldn't be letting her followers keep the portfolios they stole, she'd eat them and steal the portfolios for herself. In fact, an ultimate victory for Shar is bad for everyone, be they mortal or deity. Shar doesn't want world domination, she wants world destruction. She wants to destroy absolutely everything so she can go back to sleep.
You have cyric block the remote sensing of finder so finder won't know if his followers are being slaughtered by his chosen of cyric. And the Chosen are not just loyal they are blind fanatics... you can't reason with them they are there to only serve cyric's will and there own, but cyrics will always comes first.
Now as your stated the shar. Shar is Neutral Evil not Neutral Stupid, she knows her power comes from followers, killing your followers is like chopping off your ball-sack, you just don't do it...
and Ao didn't have to approve the potential stealing of Mystra's portfolio from Karsus' Avatar spell?
(lets find an anology here)
To find balance on Toril is like finding life in a sealed vaccum (space)
And yes you said UPON Ascension, but using that magic doesn't make them ascend it just makes it so they are mortal with that portfolio stolen from a god.