If the US gains a significant foothold there, the whole purpose of CPEC will be flushed down the toilet. Not only would they have failed to create an alternate route for transport of goods but to rub salt on the wounds, they will be handing over a brand-new port, constructed completely with the know-how of Chinese firms to the Americans. This is like a man (China) who simped for a girl (Pakistan) by buying her whatever she asked for, all with the ulterior motive of banging her only for her to ghost him and friendzone him because her richer sugardaddy ex-boyfriend (USA) and she patched up lol. In other words, China will be absolutely livid. I'm patiently waiting to see what Xi will do in this tardrage and the glorious meltdown that will take place in Chinese media.
Iran's waters are already dominated by the US and has American troops stationed in Iraq. They share a rivalry with Turks and by extension, with Azeris. Afghanistan and Iran have border conflicts that neither side is willing to compromise on. So far, only Armenia and Turkmenistan have somewhat warm relations with Iran, with Pakistan being rather neutral. If the Americans choose to station troops in Gwadar/Balochistan or choose to build a navy base there, then there is one more headache for Iran. Pakis will have succeeded in surrounding itself with hostile neighbors on all sides.