It wasn't "just" Arcade mode, but it was the most glaring omission. Like, that's the bread-and-butter of countless fighting game players. It's how many of us spent decades playing the originals, pumping quarters into the machine to climb the ladder to beat the boss and get the ending. It was shocking the game launched without an arcade mode, so it was an easy target because even the poorest fighting games still managed to include one over the years.
There wasn't even a good "vs cpu" mode at launch either. No story mode at launch. The "story ladder" was mind-numbingly easy and the art at the end was rushed and sloppy. For solo players, there was almost nothing to do but Survival mode... over and over and over again. It was a drag.
So, yeah, "Arcade Mode" isn't the biggest deal-breaker of SFV's launch, but it was certainly its most notable missing feature. I'm happy to have a mode I and others assumed was pretty much mandatory in the genre come back.