I disagree. I think yall are forgetting one fundamental point that Uhtred made in s4. The Danes would never be successful because there was no loyalty, and no true brotherhood between them. I forget exactly how he put it, but when Bloodhair and Haestan wanted Ragnar to join their army, Uhtred told them it would never work because the Danes would eventually start fighting among themselves. And thats exactly what happened. Cnut put a hit on his own cousin and Haestan killed Bloodhair.
Uhtred knew from jump that the Danes would never be successful because there was too much instability among them.
So if he betrayed Alfred, he would have been with the infighting Danes while Alfred and the Saxons would never have forgotten and would have made a move on them sooner or later when the Danes decided they were ready to go to war again.
Uhtred's best bet was always with the Saxons. And on some level, he knew the Danes were wrong. He grew up with them, counted himself as one of them and in all the ways that mattered, he was.... except for their bloodlust and craving for war.
Danes want to kill Saxons. Uhtred didn't want to kill Saxons.
So he never would have been able to go along with that.
His best bet for doing what yall are talking about was when Alfred pushed him too far after his wife died. That was his moment to join the Danes if there was ever any. And he was content to do that.
Until they started talking about killing Saxons. Then he was like, "aight, imma head out
"