Sure, I will clarify my thought process.
While I am a F2P player, I understand the idea of FEH pass. I know it's a gacha game and they have to get money somehow. For this reason, I accepted it at first.
All the rewards for buying FEH pass were nice, but nothing game breaking:
Quality of life features? Sure it sucks to have them exclusive to those that have the pass, but it only affects the maps vs the AI of the buyer.
Resplendent units? The stats boost is nice, but unless you have +10 this specific unit and if this unit is even good, it won't matter too much compare to some of the newer units.
Extra Summoner Support slots? Again, it's a stats boost to a specific unit. It might matter, it might not. It also doesn't affect for things like a defense team or arena.
Extra resources? Most of them don't have any impacts. The only things I could think off is the extra orbs and perhaps the extra dragonflowers. Plus for the orbs, they don't guarantee you will anything out of them. You are still dependent on luck.
With that in mind, why is one "free" hero so bad? It's just one hero, right? Well yes, except this hero could be a powerful hero, exclusive to legendary and mythic banner. I know that it has been a few months, but I need to remind you all it took was just one copy of Fallen Edelgard for her to completely break the game for a few months. Oh sure, now we have many (5 stars only) counters to her. At least, anyone could have their own Fallen Edelgard if they just sparked to counter her with your own. So what would happen if not everyone could spark? Well, I could tell you it would be a big advantage for the non-F2P players who rely on luck or spend hundred of orbs, ruining months of saving for a single copy... IF they get it that is.
Also, it could be another supportive Legendary or Mythic Hero like Legendary Azura but worse, who you really only one copy to make it work.
The banner this mechanic is introduced is bad because the new hero is basically copy pasted from a previous one except in a different tome colour. Wait one month or two or three and you will see how bad this change is for the game. You've seen already how IS can ruin the game with the introduction of a Legendary Hero or Mythic Hero (Legendary Sigurd, Nott, etc) who don't need merges that badly or even seasonal units with powerful weapons like Bridal Catria who again, doesn't need merges because she's a support unit.
This was my thought process.