You're a line of coldhearted realpolitikers!!!
The ancient sources all agree that Antony authentically loved Cleopatra, actually so consumingly that it rose to the level of a tragic flaw. I like to imagine that there is SOME truth to that!
Would keeping Cleo around get Antony in trouble, domestically or even from her? Probably, yes. Antony's strengths were passion, loyalty, and dogged persistency, but he was never had the makings of the truly great, of a patrus patriae.
I think the Republic may struggle along under him because he doesn't have vision or strength to implement anything really different. When he passes, naturally or otherwise, it's anyone's guess whether someone (Ceasarion, maybe) has the strength and ideal position to refound Rome in his image; or whether the Senate had reasserted itself.
After all, making Ceasarion a "king" or something like it won't be as easy to disguise as making Octavian princeps senatus and imperator. The Romans constitutionally hated monarchy and especially dreaded Eastern despotism; Caesarion will inevitably conjure up images of both.
So no, of course the pre-Marian republic will never come back. But whether Octavian's style of empire is inevitable, I have to emphatically say no. It could be worse, or better, depending on the circumstances.