In Manly's defense, you have to say that compared to ANZ Stadium - which is more accessible via public transportation - Brookvale Oval is a big problem to get to for a Monday night match. It's unfortunate for them that Pittwater Road is the main route to get there from the city. Hold last night's match on Friday night, Saturday night or Sunday and with good weather, you can add another few thousand to last night's crowd. Irrespective of whether some grounds are easier to get to than others, I think we all agree that across all teams, Monday Night Football is a crowd killer and whilst the game is great to watch on tv, it sucks for the team hosting in that timeslot. Not even the $50k or so that Gallop introduced to compensate teams for hosting matches makes up for the lost revenue in crowd attendance and other facilities like food and beverages.
However, i do agree with you that the Sea Eagles will be in big financial trouble if they don't get any upgrades and much needed corporate facilities to their ground within the next 5-10 years, and it won't be financially sustainable to continue playing out of Brookvale. For the Sea Eagles (and this has been discussed in their forum), they'll either have to take more home matches to Gosford, Allianz, or if it was redeveloped, play matches at the stadium in Narrabeen (if they want to remain in the Northern Beaches).
I can see with the recent proposal for Allianz Stadium to be done up and ANZ Stadium paying teams to play their home matches there, Sydney teams that play their home matches at suburban grounds will be taking more home matches to Allianz or ANZ Stadium. It's inevitable.