Penrith and Parramatta (separately) are in key areas, given the AFL's planned expansion into western sydney. If needed I can see the NRL helping these clubs survival as stand alone clubs on a strategic basis if nothing else.
Wests Tigers also have important territory under the banner, including the population growth area of Campbelltown, where they really need to be playing more of their games in future years imo.
St George Illawarra is already strecthed in covering a very large area, including the traditional league nursery of Illawarra. Let a presence in Illawarra die out, and the league stands to lose a lot.
Similarly Manly now have the lion's share of the north shore, and strategically, geographically, are very important to the league's presence in all demographics of Sydney.
Easts are an important presence in that part of town, and are our last remaining continuous foundation club. Can't see them moving or folding, even if their catchment area is small and juniors aren't their strength.
Which leaves Souths, Cronulla and the Bulldogs... As for Souths, it seems like they are doing good stuff with the branding and revitalising their Redfern facilities and involvement. I didn't think Souths and Easts could both survive and stand alone purely georgaphically, but Souths' junior area, their re-entry to the comp and their achivements since seem to be placing themselves in good stead.
Bulldogs have a strong brand that could take root if relocated, and essentially have already left their traditional geographical territory anyway.
Cronulla have a pocket of land in-between the two arms of St George and Illawarra, and reportedly have a few more finacial woes than the average club. A relocation and keeping a Shire team in the lower division might be a good answer to a problem.
I don't want to see any Sydney team forced out ahead of their time, but under consideration there are two more obvious candidates for eventual relocation - Cronulla perhaps by necessity of things don't pick up and return to the level of interest in the Shire the team had ten years ago - and Bulldogs perhaps because the club is ambitious enough to be proactive and want to shift.
I think all other Sydney teams need to stay. You can't just kill off bits of the game for fun and think that it will survive undamaged. Wait until they go broke (Newtown, Norths, Magpies/Tigers etc) or put relocation down as potential business growth idea. And wait until the game can sustainably afford to bring in brand new teams from areas that don't want a relocation... the game had 20 teams once, before Super League came and wrecked the sustainable exapnsion agenda. Once we have caught up the damage done, we can (eventually) get back to 20, without needing to cull Sydney teams down to 5 or whatever and lose (more) fans.