Ah bummer. Was somewhat of the view that MAX level would be a bit more for her (as you would plausibly be able to beat her battle forms of course not at full power, but still notable, at that point). The notion that one could use it to resolve what Ancilla discusses how disillusioned/detached she is compared to before (by playing into her desires she is restraining, and its some how "more" to her if she becomes attached to you vs random people she isn't?).It seems unlikely that Sofia will be included in the Level Drain system given Sofia's existing drain scene. In one of her defeat scenes, Sofia will drain all of your stats and levels, stopping just before she drains your soul, just to state that they do not really do anything for her before she eventually returns everything she absorbed. It is quite likely that Sofia is completely maxed out given she has devoured millions of souls, and likely the stats and levels of a number of her victims as well. She is clearly much more physically powerful than her appearance and currently revealed stats may suggest given she is able to crack the floor of the Forest Dungeon with a single stomp during her Collar scenes. While Sofia does state she is not quite as powerful as she was when she was devouring souls, rather than having directly lost power, it could be a situation similar to Arcueid from Tsukihime and having to use a portion of her power to restrain her soul-thirst so that she just does not go on a rampage and start consuming everyone before her given she stopped soul-eating cold turkey.
If anything, Level Drain will probably be specific to a future NPC, and perhaps a few other characters. That way people who enjoy the content can freely indulge in it while those who do not will not have to worry about their efforts leveling up their character being undone. Just like those who enjoy grinding to a degree will be able to use the Rebirth system to continually grind and power up their character somehow while resetting their level to 1 each time they hit the eventual level cap of 100.
While this may be disappointing, I do hope that you find this enlightening, Gojii.
But oh well. Is there going to be some sort of content to address what Ancilla says about "helping" Sofia's emotionally detached state? That was more fo the reason for it. Though it would play into it well, but guess not. Still curious if that is something that is a plot route with Sofia (to learn the "real" her).
What of the concept regarding Feng? (Even draining spirit in lieu of levels could be interesting for either). Seems like Kotone is really remorseful. Willingly putting yourself/levels/(or a certain threshold of spriit being harvested over time), to help some crazy plan to restore Feng (maybe a late game "book" you find in the Labyrinth, which causes a dialogue option with Kotone to help Feng, like some sort of banned/lost spell/ritual?). Either way definitely going to delve into the new version.