I get what you're saying. It is an identifiable brand but on market research that I've seen previously it's often listed in the bottom 3 out of all the Sydney NRL teams in terms of brand awareness. So it's not as strong as what you think it is. If it stays as Manly and Manly do little to engage the North Sydney area, then the Sea Eagles will likely always be viewed as a 'Manly' team and likely of less appeal outside of the Northern Beaches. In which case, Manly's fanbase will continue to come from a very slowly growing and aging population, compared to say the Panthers and Eels, who are going to basically be splitting up the million or so who relocate out west.
That said Super League was a generation ago. Boomers will hold a grudge but the kids in North Sydney that you want wearing a Sea Eagles jersey just want to feel like it's their team. They have no idea about that history. Their parents will take them to games if you give them a reason to want to.
It’s a stronger base to build from than scrapping it for a new identity. Direct engagement with an NRL team is what the North Shore needs (and currently gets basically none of), growth of support would naturally flow. When looked at together, the combined North Shore + Northern Beaches base is big and pretty future proof, despite the rate of population growth being slower in those two regions. The amount of wealth and influence in those two areas would more than make up for it and make for a strong NRL team.
I think North Shore would be a better name to use to include the region. North Sydney’s been used before in a RL context and to most people from Sydney is used to refer to the suburb of North Sydney or North Sydney Council area. It’s sometimes used instead of North Shore to refer to the whole region, especially in RL.
The problem is North Shore as a name on its own doesn’t include the Northern Beaches, neither does North Sydney- the name is very tied-in with the North Shore. That’s why combined sports comps that are played across both regions are often called something along the lines of ‘Sydney North’ ‘Northern Sydney’ etc- calling them North Sydney generally isn’t done as people associate it with the suburb and the North Shore region.
How can you include both regions in a team name though without cutting a region out or using a generic Northern name? Copy ‘The Dolphins’ and potentially annoy/alienate the existing Manly fanbase? Or do it the St George Illawarra way and have both regions in the name?