i agree that brehs need to let it play out a bit.
as powerful as MCU-dormammu is he cannot understand or fight time.
who is to say what limitations the celestials have or do not have. maybe they cannot sense or manipulate across or outside of timelines - thereby giving he-who-remains (and his potentially endless variant manifestations) a cockroach like tenacity that even they cannot control.
maybe for all their power the celestials are entirely predictable. again given he-who-remains an insurmountable advantage.
if kang can manipulate others to branch the timeline then surely he can branch it himself. if the celestials came to destroy earth he could just split the timeline and move to one where they don't destroy earth, rendering the effects of any celestial destruction in the branched-away-from timeline (for him "kang") irrelevant.
if you knew everything in advance, could travel through time and across timelines, could exist outside of time and could manipulate time itself who could stop you? and by "you" i don't just mean this version of you but rather all versions of you.