Sabo was introduced out of nowhere, just to be killed in the same flashback, and suddenly after coming back, everything else never made sense. Known as one of the main figures in the revos, but apparently no one including Ace knew he was alive. Himself conveniently having an amnesia to explain why he didn't come to MF, but conveniently once again appearing again after the timeskip when Ace already died to take his devil fruit, and suddenly wants to follow Ace's will
That alone implies that not only Sabo was never a character planned at first, but his role in the story was mainly to cover Ace's absence after his death. In his youth at least, he used to be more calculative, but now he is completely reckless while under pressure, reminiscing a bit of Ace once again. Just look at his reaction while infiltrating the reverie with other revos telling him to calm down, but not wanting to listen and opting for a "do or die" scenario to save Kuma eventhough Dragon wasn't even personally involved in the mission yet. As a result, Oda is seemingly once again going in a Ace/Marineford 2.0 with him apparently in danger
On the other hand, Ace was introduced since early on in the series, with Oda taking time to develop his bond with Luffy, hating on his bloodline and wanting to know the reason behind his existence, if it was worth or not, with the goal of becoming a pirate king, just an indirect mean to his true goal, of making himself known and recognized by everyone. Whitebeard in a sense was viewed as the father that he never had in his life
And in their youth, Ace had the edge over him in their fights, unlocked conqueror haki which Sabo isn't known to have, known to have pirate king potential which Sabo isn't known to have, with entire Blackbeard pirates Burgess included being portrayed as fodder in his battle with Blackbeard, same Burgess who was portrayed as a match for Sabo in dressrosa, showing that Ace had better growth rate, so sure Sabo had an additional 2 years over Ace to grow stronger, but overall in terms of characterization, power and potential, at the moment he looks more like a cheap version of Ace
And i don't dislike Sabo(i want him to own that overrated Sakazuki who is like the only character i hate). I just don't care about him much, or at least not like Ace. I actually think he could have been a better character if Oda continued with his development of Sabo when he was younger with him revealed to be alive before Ace's death, because now he just look and will continue to look as a replacement to cover the fact that the original(Ace) died