I don't blame them since making Mario sports games is more efficient and brings in more revenue. A new Golden Sun game would probably take Camelot three years to produce and sell 1-2m if things go right while they could probably make two Mario Sports games in four years and get 5m+ in sales. Mario Tennis Aces alone might end up outselling the entire Golden Sun franchise.
I agree that the Mario Sports games are a lot safer bet but I don't look at it that way in regards to whether Golden Sun should make a comeback. Golden Sun 1 sold 1.76 million on GBA and Golden Sun 2 sold 1.22 million, that's pretty impressive for a new franchise on the GBA especially since the 2nd was released pretty late into the life of the console (not so late that it was irrelevant mind you, but still). After games like Fire Emblem Awakening and Xenoblade Chronicles 2, and after every Nintendo franchise has seen a huge increase on Switch ... comments like these are a bit head-scratching. They can always fill a niche in the Switch's library gap, and since Nintendo games generally don't have a big budget they don't need to have long development times. Although I think the problem is more so staff numbers ... Nintendo would probably just give it to another studio or hire more staff for Camelot. Either way, I don't think Nintendo will do it.