It depends.
Shayna can actually work. She's OK on the mic but it's the matches that tell the story. Thing about Shayna's reign compared to other champions is that she really only specializes in one type of match - the big dominant heel that methodically breaks down opponents till the face fights back. Tried, tested, true. But it can make her matches 1 dimensional, no matter who she faces.
Alexa on the other hand isn't a good worker, and isn't good enough on the mic IMO to overcome that IMO. She can work a very limited kind of match and doesn't even bump all that much. Only wrestler I've seen get a good match out of her is Sasha (ironic, given they don't fukk with each other).
I think Shayna would have been an improvement but heel champs hogging the belt is played out.