Hmmm yeah thats a interresting theory, i always got the feeling alice is expected/supposed by her ancestor (and maybe even the fairy) to kill the fairy too and take her/his place.If we think about it, could Mother want the same from her children? Mc's mom said that Mother was disappointed with her, maybe 'cause Mc's mom don't want to surpass Mother and choose a different way which ended disappointing Mother and I think the only way to surpass a monster ancestor is to kill and devour it!
There's a webnovel with the name Warlock of Magus World (maybe someone here already read it), where the mc of this history ended up becoming a Magus that use a bloodline of a monster to achieve higher levels, but after reaching a certain level he discovered that the ancestor of that bloodline had control over him and to free himself of it control, he create a plan to kill this ancestor and assimilate it's power becoming the new ancestor of that bloodline. Maybe Mother wanted that to happen ? Or all apostles want this to happen ?
And yeah Syla may had the potential to surpass the fourth and just lacked the ambition/cruelty and prefered being a mom.
If this is true, than Syla is about to prove them all wrong (dexter for example) as she is her sons guide in a supporting way instead of shapeing him to overcome her.