“If Ace had haki why didn’t he use it against Blackbeard?” Who says he did not do it? Remember haki was always invisible pre-skip, the only way we knew someone used haki was if they faced logia or it was directly stated in the dialogue. Blackbeard is always tangible because of his Gura Gura no Mi and his crew evacuated the area so there was nobody else to comment on Ace using haki (also remember a lot of the fight was off-screen).
“Ace got fodderized by Akainu” Ace had been tortured repeatedly in Impel Down and was given little to no food or water so he was not 100%.
Other Ace’s feats:
–Offered title of Warlord at 17 years old! People will try to invalidate this by saying Buggy was offered it as well however Buggy got it because of his reputation of being linked to a Yonko and to the Pirate King, the Government wanted to use his reputation to scare off pirates, while Ace had no reputation as a rookie, he got it purely because of his strength.
–Clashing with Aokiji and stalemating
With all the feats I stated here it’s safe to say Ace is like “low tier Admiral level”, strong enough to halt and injure an Admiral but not beat one. That’s why Ace had to be killed off, it’s because he’s much more talented than Luffy and he would have become the Pirate King if he reached his potential (Whitebeard would have made him).
*Theory by Chari_D_Tbator