It does not necessarily be a 1 to 1 replacement, which was the case when 77W replaced 744, or 359 replacing (some) 333 & 77W. So long as it works for the overall fleet, a mix of 78X and 339 (or even 321neo) can do the job well regionally. Plus CX has been putting long haul planes on certain regional routes anyway.

Not a big fan of Boeing or 787, but just that CX may be putting too many eggs in one basket if they only operate 77X from Boeing, but 321/333/339/359/351 from Airbus.