Comparing LAX’s ops to JFK, that will hopefully be sufficient in terms of capacity. JFK has dozens of long haul flights leaving within a few hours of each other (+JFK and SFO). I think DL just has 1x: CDG, HND, SYD, AKL, BNE and PPT (and some of those are not daily) + JFK?
I can see VS passengers using the new lounge until the new Clubhouse is open (and for some, even after) since UC check in is now colocated with DL, but for JV partner passengers, it will be a bit of a hike so many may not choose to walk all the way over.
I think it will be plenty of space. As you point out, not many Delta One flights out of LAX - just those long-haul destinations once daily plus JFK, BOS, DCA. I'll be interested to see if Virgin Atlantic Upper Class guests choose the Delta One Lounge or the new Virgin Clubhouse at LAX when it opens. 10,000+ sq. ft. is still quite quite large, about the size of the FLL Sky Club. The LAX D1 Lounge will be only a tiny bit smaller than the UA Polaris Lounge at LAX (noting they don't allow transcon/GS passengers), and larger than many other regular clubs in the U.S.