Oh okay I see what you're saying here. You're saying that 9 biju chakra pieces together automatically allows you to gain the senjutsu that the jyubi because the jubi has only senjutsu chakra ,right?
Well one thing I need to mention is that the jyubi has chakra and senjutsu chakra because without the basis of chakra you cannot gain natural energy on top of that. So there is jyubi chakra and then there's jyubi senjutsu. With that said. All 9 biju chakra still needs an additional senjutsu to manifest rikudou senjutsu. Kaguya aka jyubi can gather NE in addition to its jyubi chakra where as Sasuke's Gedo/Biju susanoo cannot. You agree, right?
And if you also think about it, if Sasuke's susanoo can use senjutsu then Indra's arrow would've overwhelmed naruto's ashura shurikens.