So you're assuming space-time ninjutsu has no drain on chakra whatsoever? So you're assuming Minato can simply use FTG as much as he wants? No toll, limit, ramifications whatsoever? Wrong. All techniques use chakra (manga fact), and Minato's FTG is limited. He doesn't have an infinite supply of kunai/tags. That's just not realistic.
And based on manga, I'm pretty sure Minato would tire before Sasuke. Unfortunately for you though that isn't the only battle scenario. It most likely wouldn't be a fight to see who outlasts who because Minato has a limitation of FTG. Manga fact once again.