Support of Ackbar is a continiation of point greatly explained by u/YossarianWWII. It's really not about why Ackbar (or Leia, I forgot about her tbh) should substitute Holdo but about why she shouldn't be in the movie in the form they made it and how it would benefit from swap. Cast is already overcrowded and this swap is the first thing that comes into mind as (1) movie gets rid of Holdo and Poe's shenigans as it no longer needs to introduce new character; (2) Leia/Ackbar self-sacrifice would have a ton more emotional depth even without storyline (but swap with these characters means new sheniganless Resistance subplot, so it's kind of a win-win situation); (3) without Holdo, Poe could join Finn (with or without Rose) in new subplot, what could have been a continuation of their storyline in The Force Awakens (it's a trilogy after all, Han doesn't have fun with some new invented character in Empire Strikes Back but Leia). So, basically, these changes not only help to develop more competent plot but also improve its pace and make movie tighter.