I also disagree with it replacing Sidisi, she's a top tier commander for a reason, she kicks ass. The commander may replace Kess tho. Seems better in every way, and has green instead of red
Sidisi is top tier, but people get tired of playing the same deck and change commanders because they want something new (I did with Daxos changing it to Azor after having Daxos built from the time he came out).
Personally, after I complete the Chromatic Challenge, I'll probably make a Muldrotha deck but keep my Sidisi deck as something to play occasionally but Muldrotha will likely be my primary Sultai commander.