A359 will be a massive increase in seats vs an A343. After the shock of upgauging a number of routes from A343 to B773 I doubt LX will get such high seater...
During the 2014 announcement of B779 and A359 purchases, LH showed this image:
Which shows a clear gap between the A330 and A359 where the LX and OS longhaul fleet with 250-300 seats needs replacements. This gap is more the home of a B789 or A339.
If you look at United's recent investor presentation is becomes clearer: