There's no retcon, Luffy can only do so much. He already used way too much stamina before his fight with Kizaru, it was emphasized. Luffy used Gear 5 twice this arc, which is the same amount of times he used it against Kaido. Whether it's consecutive or not isn't relevant. In fact people were even questioning why Luffy was so tired back when he ran around the lab. It was also stated that awakening is a heavy toll on one's stamina. Luffy can't just "keep restarting his heart."
If you can't keep up it's okay, maybe try the very hungry caterpillar, it might be more your pace.
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Title. Imo its a good thing because it keeps fights tense against other top tiers. Infinite G5 would be pretty broken.
Like, why the fuck are you acting like infinite gear 5 was implied by Oda anywhere in this manga?