If the songs are going to be played with the mindset of 'This is Crowded House' rather than just as 'Songs of Crowded House played by Neil + Others including Nick', then I think that we are very lucky.
I thoroughly enjoyed the November 2016 Sydney gigs because the band sounded fresh and vital when the 4 of them were together...it reminded me of seeing the band in March '92 when Mark had replaced Tim; the songs were breathing their own lives with each performance. Beautiful songs outlast the life of any band, and Crowded House has a surplus that is the envy of so many musicians (why are there so many tribute bands plugging the holes in venues these days?). This is not to distract from Mark's role or Neil's decision; they'll talk more when they're ready...the dark horse in all of this (for me) is Nick Seymour.
Nick's style and approach is the glue between Neil and everyone who has been in the band in my very humble opinion. The 7 deluxe sets highlight this in spades. Neil is exacting and very particular about who he works with over a long period of time; Nick outlasts just about everyone in this regard. Neil's current international profile will undoubtably reflect well in terms of timing the return of Crowded House; while I'm saddened that Mark won't appear at Byron Bay, I'd prefer to wait until further insights are shared IF it's appropriate for the public to know.
I don't think that Mark can be replaced when considering his harmonies, lap steel, keyboards, guitars, and harmonica prowess....a new musician will have to carve their own niche. I am however intrigued and very hopeful that Crowded House will outlast a temporary visit next year.