If Easts Tigers were in the NSWRL for 100 years, fell into decline and were replaced with Balmain who modelled their very identity on that of Easts... jersey, mascot, logo... You'd definitely get some, purely for nostalgia. I didn't say heaps, I said some.
In the unlikely and completely hypothetical situation that Wests demerged and Brisbane Tigers came in, they wouldn't rely on Balmain fans for support, they'd have plenty in Brisbane, but there'd certainly be a higher than usual support in Sydney than for other new QLD teams like Tits or Cows.