Wow, that is a hell lot of pessimism right there. You all right man?
To be honest, Golden Sun in a way is in a better position than it was before Dark Dawn. We had to wait 7 years before Dark Dawn came out, and the ending of the Lost Age was much more conclusive than Dark Dawn. Even TLA was unable to top the sales that the original Golden Sun got AND it also performed less than the original in the market.
The franchise wasn't dead then so I don't see why it would be considered dead now. Nintendo is known to pull out dormant franchises and make a game. Kid Icarus and now Star Fox, say hi. The creator of Golden Sun has expressed interest in a sequel to DD. It would have already been released/announced if DD had better sales but since it undersold (still more than FE used to sell before Awakening), I can understand their apprehension. They have to be careful for the franchise her on.
A franchise is dead when it said to be dead. Mother is a dead franchise. We do understand that Camelot is nowhere near its glory days of GBA where it pumped out great games. Golden Sun: TBS and TLA, Mario Golf and Mario Tennis: Power Tour each with full fletched story modes (RPG style), and many more. I can see how the current Camelot might have a hard time making a new GS as they have also lost some staff over the years.
BUT Nintendo is less averse to trying something different these days. In the older Nintendo, so much attention wouldn't have been given to Xenoblade X or Splatoon (although they still undersell games like W101 and Codename S.T.E.A.M). They have also helped Monolith Soft in some components of X. If GS picks Nintendo's attention (again, it was already a big reveal in E3 2009, hence noticed by Nintendo) it is very likely they could help Camelot make a new GS game. 3rd party exclusives like Bayonetta 2, and Devil's Third show that Nintendo is now more willing to help out, and GS is a Nintendo franchise.
And regarding the absolutely lack of Golden Sun, it is not that bad. Golden Sun still maintains a presence through 2 soundtracks. Sure Isaac's assist trophy might be gone and he didn't get a regular trophy, BUT the lack of AT is a godsend for us Isaac supporters. The ones who are already ATs virtually have no chance (or an extremely low chance) of getting in Smash 4 DLC ballot or not. Isaac on the other hand does have a good hand.
He has been topping recent community polls or coming in the top 2-3 if you count out Wolf. He was even 2nd in the recent Reddit Poll (after Banjo who sadly has an extremely low chance - next to nil). Isaac's popularity can be seen. Even if he doesn't make in Smash 4 DLC (although his chances are much brighter now - may the Golden Sun shine upon them) Nintendo will know the demand for him.
I think Golden Sun 4's chances are not zero as you might believe, Isaac gets in Smash DLC or not. Although we might have to wait for a bit longer. I don't think he is a shoe-in but you are definitely underselling his chances or the chance of him getting another game. Dark Dawn was successful, believe it or not, just not upto its Golden expectations. It might have screwed up the story a bit, introduced over-complicated things and turned off some GS fans but it hasn't killed the franchise.
The Sun will rise again! After all, the eclipse is over