I think 'will be'. Personally I want Kean to stay with us, but he needs game time to help him develop an understanding with the rest of the players and build his confidence etc. Obviously Ancelotti knows of what type of forward he wants at the club, and it could be that Moise Kean isn't.
I feel like I might be a bit ignorant here on some of the top level football management stuff, but it seems to me like when you have a top-of-the-heap manager like Ancelotti he probably has an extremely clear picture of exactly the types of players he wants in each role and how each piece works together. There may even be extremely good players who do not fit into that picture. Meanwhile, when you hire a relegated manager who was then sacked a few months into his next job because his side had already started falling off, that person probably does not actually have all that clear of an idea what he's looking at and what he actually needs.
The impact of that might be that when a DoF is looking at potential signings, the one who is not all that great goes "Yeah, sure! He played for ____ and seemed good, so bring him in; I need more players," as opposed to the better one going, "Wait, how does that fit in with how the rest of the side plays? Why not sign ____ instead, or somebody who [insert characteristics]?" That would help explain why even the presumed non-Ancelotti-initiated signings already appear to be doing a lot better than their predecessors from this time last year.
On Kean specifically, I really do think he's going to have multiple 20-25 goals, 5-10 assists type seasons in his career, but it might just not be in the cards for him at Everton. Kind of like how Immobile was completely unproductive at Dortmund, yet extremely productive at both Torino (before) and Lazio (after). Some players just fit better in certain situations.