If your chakra is strong AND large enough, you can attain PS regardless of having MS or EMS, and that's why EMS users and MS users with Rikudo's power can get PS, because of the extra boost in power that EMS grants, or the extra boost in power Hagoromo's chakra grants.
But due to how things are, there will be no MS Uchiha capable of awakening PS w/o an outside power boost. None. Madara has the largest chakra the clan has ever seen and still needed EMS to reach the level of PS.
Completely agree with your post (aside from 2 points stated below).
@bold "Strong enough" is okay. But why "large enough"? I mean, sure you need large chakra reserves to manifest and maintain Susanoo, but I don't see how chakra quantity is related to Susanoo's progression/upgrade. Because Susanoo's progression is reliant on chakra quality/power. That's why EMS users and MS users with Rikudou chakra can get PS, because they receive the extra boost in power that EMS grants or the extra boost in chakra quality that Rikodou chakra grants.
@underlined You cannot "attain" PS without MS. You need MS for manifesting Susanoo. But once it is unlocked, it can be used and progressed regardless of MS and EMS.
@red Simply because Susanoo's progression is independent of chakra quantity.
Then how do you explain ems sasuke susanoo growing legs the first time he brought it out after receiving naruto kyuubi chakra and not before.
When did EMS Sasuke grew Susanoo legs
after receiving Naruto's Kyuubi chakra? Do you have any scans? Because I don't seem to remember that.
But let me answer this anyway. Susanoo naturally progresses as the user gets more trained with Susanoo i.e. constant usage. So as time passes, Sasuke's Susanoo will grow stronger regardless of anything. And that's why it manifested legs, then it manifested PS form.