Shirley never worked as a replacement for Len but she's gradually got worse each year to the extent that she's now a total liability. I know many on here (myself included) find Motsi & Anton annoying, but both at least get that it's entertainment and aren't really detrimental to the show. 99% of viewers and seemingly many contestants can't stand her, she's rude, biased, extremely publicity hungry, continually drags the atmosphere of the show down and is paid way, way too much money funded by licence fee payers (that part is what really gets my goat).
In short - If the show wants to continue being successful, they have to sack her. But they seem afraid of her being Arlene 2.0.