well things are hardly over for Jon and Dany. I mean, I *hope* Cersei wins in the end, and that Dany and Jon are both destined to be nothing more than the next fan favorites to die. But let's be real... it's really, *really* unlikely that they lack plot armor in the ways that the dead Starks and Oberyn did. Dany and/or Jon may still die before it's over, but no way would it be very much before the entire season finale. Early in season 7? Naah, we'd have to see someone else step in to be the leader who rallies Westeros against the Night King. Too much narrative wasted in starting over for that at this point. Besides, they just killed and brought back Jon. If anyone's unkillable, it's him.
Things are going great for Cersei and terrible for Dany, yes. Perhaps the trajectories are such that it's going to be diverging fortunes even more sharply before they correct and converge towards one another once again. Because afterall we've been hearing a ton about how invulnerable the Dragons are.. which pretty much means we're gonna see some more bad news on that front to prove how invulnerable they actually aren't. And while Cersei's on a high note with paying off the Lannisters' debts to the Iron Bank with the wealth of House Tyrell... it's also just a bit TOO good. Something bad is due to occur soon. Perhaps Olenna's revenge from the grave will be souring Jaime on Cersei once and for all. Or you know, perhaps Euron will just betray the Lannisters. He's almost singlehandedly responsible for Cersei's successes this season, and if she tries jerking his chain she might not be able to handle him.
Oh I have my own personal meta theory about Tyrion that S7 is certainly supporting thus far: He's there to ruin the schemes of everyone around him (so was Cat... they were a great pair early on). He's been a living monkeywrench in everyone's grand plans ever since season 1, and this season certainly is making Dany look like she would have been better off not listening to him. Tyrion means well, but he's the source of more chaos than Littlefinger himself...