As long as the Sydney based teams are spread out and aren't overlapping or congested in certian areas of sydney there's no reason they cannot all be there, trouble is there are clubs that have areas that they doing nothing with, or clubs that have bugger all areas and just poach heavily off others, both not really advancing the league in any metrics regarding memberships, junior participation or crowds.
People generalise certain clubs, and that end up getting a media stigma, which everyone flocks too and uses them as facts, which is incorrect, just because a club is successful in premierships and consistency in finals doesn't mean they are vital to sydney or the league overall, ive always said two clubs i see as useless are roosters and manly, nice to haves, but not nessesary.
If they both go thru rough patches they won't keep up with the rest of the metrics of the league, where clubs like Parramatta or Canterbury can be on a drought and still service there members, juniors and catchment regardless. this is more valuable to the league than winning on field