I don't think it was. How do you explain characters like Ace and Crocoboy not having Haki, despite them being New World Veterans? I believe Oda most likely intended for weaknesses to be figured out for each Logia, like it was for Crocodile, but decided to create haki to make fights more convenient and Logia users less overpowered. Keep in mind, One Piece wasn't supposed to go on as long as it has, so it's probable that he didn't have haki in mind since the start of the series.
I doubt we'll get a flashback of Ace using haki because, again, it would create a plot hole.
Ace there's argument for that he possibly did.
On behalf of Crocodile, he definitely didn't possess Haki or Luffy would never have been able to defeat him so cheaply. He showed no ability in CoO (at least as Enel did) and Luffy had no issues whatsoever hitting Crocodile which eliminates CoO automatically imo.
Luffy would have referenced Crocodile having any form of Haki when Rayleigh was explaining to him and he only referenced Enel, Sentomarou and the Kuja Warriors. In my opinion, all the evidence points at Crocodile not possessing any form of haki.
As for Ace not having Haki, there's no evidence of him using it as a grown up which is a shame (besides when he was a kid where he released CoC). On the other hand, there's nothing that explicitly hints that he didn't possess other forms of Haki too.
I think one of the problems with One Piece concerning this topic in general is that we know that to hit a Logia one needs CoA, but that's not the case vice versa.
If we see a normal being hit a Logia pre time-skip or not, we can automatically conclude that he/she has haki; whereas when a Logia attacks another Logia, we just can't tell which leads me to believe that Logias don't use CoA possibly because it negates the effect of their DFs.
Smoker is obviously going to come up based on what I said about Logias not using CoA, but he's different to other Logia in the sense that he's not going to smoke anyone to death. A punch of smoke would at worst make one cough, the result is different in the case of Ace, Aokiji and Akainu.
Smoker's DF seems to mainly give him an advantage in being intangible, manoeuvring and landing surprise attacks. His DF doesn't pose that much of a threat imo.
When Akainu took Ace out, Marco and Vista attacked him and he groaned out "annoying haki users" which may imply Logias don't use Haki against each other.
All the times we've seen Logias fight each other we've never been sure as to how they fight or when they utilize Haki and what effects it has on them whilst they're intangible or in a Logia form.
If 2 Logias not possess Haki fough each other, how can we tell who will be the superior? So far the only thing one can assume is that it depends on if the DF negates the other or not going by all the dialogue pertaining to such fights.
Oda needs to show us how the fight between Akainu and Aokiji developed so we can understand better. Also, on behalf of Crocodile being weakest amongst all Logia (possibly excluding Smoker), I've got some evidence if interested to see it.
Apologies for typing so much