Exactly... A harem is basically when you can "catch them all" and marry them and all that... a poly relation is not the same... The only "larger than 3" poly relation I see viable right now is with those 4... mind you, I haven't played all routes (it's a looong game) but not all groupings make sense. What other viable larger than 3 poly options are there?Read ACEWINZ comment about relationships
I don't see possibility of any pairing other than Jenna/Katie/Lily/Debbie with 3 or more girls.
Regarding Katie: Katie is part of the "main girls" group and as such there is a character arc for her, much like for Jenna... Jenna's character development arc is getting out of her shell and dealing with her physical insecurities, which she has mostly completed at this point... Katie's arc is also about dealing with her insecurities (which are more emotional/psychological in nature) and she's been extremely stubborn up to this point.
While it's possible that she won't ever change, it'd be kinda lame if she never did because that would mean an incomplete character arc, and I just don't see that happening, since the writing is actually quite good.