What was the biggest mistake the producers made in the post-Kate Jackson years?

*Many of the Season 4 (the Shelley Hack year) episodes mainly center around one Angel where as in the earlier seasons, the episodes involved all three Angels. And with Shelley, her character-centered episodes didn't start until almost halfway through the season.

*Giving up on Shelley Hack too early. What I mean is that if Shelley was really that lackluster of an actress (hence why her role was limited and she wasn't promoted as efficiently as she otherwise should have been), then why was she hired in the first place? In other words, she presumably auditioned so they should've known what they were getting.

*Replacing Shelley Hack with Tanya Roberts. Basically, if Shelley's acting was a main issue then, why would they replace her with depending on who you ask, often wooden acting skillset wise (especially when they get back to LA from Hawaii, and thus, Tanya couldn't get by in a bikini) and whose credibility was arguably based solely on her physical appearance.

*Not making Tanya Roberts' character an ex-cop/law enforcement officer like the other Angels, thus tossing the whole concept off-balance.

*Not thinking "outside of the box" so to speak and replacing Kate Jackson with a minority woman/woman of color (instead of another white blonde woman to go with Cheryl Ladd) such as Shari Belafonte, who was apparently considered before they went with Shelley Hack.