Thanks.
The YM negates astral techniques [
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]. Astral: of or consisting of a supersensible substance held in theosophy to be next above the tangible world in refinement. In other words, astral is the very definition of Obito's Kamui; it is the category under which Kamui would fall, and both the databook and manga claim for it to be able to defend against such attacks. Obito will obviously be targeting the caster of the Susano'o and not the Susano'o itself, which is useless seeing as the YM will be in front of him always [
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].
Since long-ranged Kamui would be his only option once he finds himself unable to get close to Itachi, the YM negates it as it was canonically stated to be able to do.
As shown here, when a dojutsu technique fails in the process of its execution, it causes immense strain and pain on the body [
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]. Assuming Itachi has intel, he won't let Obito go anywhere near him, which is something Obito would have to do in order to perform a Kamui. However, Obito also has a long-ranged Kamui, which he will rely on in order to stop Itachi from using his Shunshin to out-distance Obito so he'd be unable to get close to him.
Kamui itself is arguably one of the most chakra taxing techniques of all time. To have it broken would cause even more strain than usual on his body, and he himself would be prone to any and all attacks. Once Obito suffers from the backlash of Kamui, Itachi simply Shunshins near Obito with an already-formed Susano'o arm wielding the Sakegari and promptly seals him.
Itachi's Shunshin feats: crossing a considerable distance in an instant [
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]-[
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], Shunshing behind Bee without him even noticing had Itachi not warned him [
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], blitzing a Sage with perfect sensing to the point of where even he was alarmed [
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] (note a Sharingan user couldn't even understand what had happened when it did).
Considering these feats, Itachi should be able to easily cross the distance in the interim of Obito's backlash and his recovery.