Although this is all speculative at the moment, I wouldn't be surprised if UA kept 3 or 4 of the IAD UC's open after PL IAD is opened. C7, C17 and D8 are oftentimes near-maxed out capacity, even after C4 was converted from FCL to a UC in 2018 iirc. I'm not sure if the 4 could be consolidated.
And LH and TK are both wonderful lounge alternatives, sure, but the walk to them from C (even with the buggy) leaves more to be desired, especially with time constraints ):