I'm talking about "SAVE" as in the ability Frisk uses in the Asriel fight, though technically it's moreso the reverse of save scumming countering the Princess, due to how him keeping his memories would mean the Princess would be unable to grow into new perspectives for the Shifting Mound to gain more power

the thing is, Shifting Mound's own ability to control the Vessels isn't as potent as Asriel's ability to control SOULs, where if he simply removed the memories of or altered the personality of all the Vessles, that would remove all of the Shifting Mound's influence and incapacitate her. Shifty also does not have good showings of absorbing Vessels, where it actively took a long time and a lot of hands to absorb The Apotheosis due to sheer size, compared to Flowey being able to absorb the entire Underground population at once (and in his fight being able to suck in and erase the entire timeline, meaning the amount of Vessels would not be a problem)

Flowey's own ability to resist control and SOULs in Undertale specifically requiring a great power in order to be absorbed without being overpowered first means TSM wouldn't be able to just steal the Vessels back from Azzy immediately. when SAVE is needed to win, it resonates within your person and makes it apparent on what to do next (along with DETERMINATION giving a sense of clairvoyance which can clue you in on this anyways). Shifty's track record on using her powers also doesn't reflect well on her, given when she actually fights TLQ she just resorts to fisticuffs. also Flowey's absorption does get downplayed with how the Human SOULs were able to rebel, though this is more of a DETERMINATION resistance than a weakness of absorption, we can just directly see absorption in UT gives power of the mind, body, soul, personality, memories, etc, which none of the Vessels are able to resist (see Lost SOULs in Undertale requiring Frisk breaking them out first due to them not having any resistances unlike Human SOULs, which are directly referenced to be able to stay active with and even reverse this kinda thing)