So your argument is "why be upset about one bad unit when you have a lot of better units", yes? Because that's what I'm going to roll with.
The guy appears out of nowhere. I don't have a problem with that.
The guy is a T1. I don't have a problem with that.
The guy is an axe main. I especially don't have a problem with that.
The guy arrives after he would've been useful, already having at least one or two axe units. That's a problem.
There's a very low early game ratio of axe users to anything else. That's a problem.
Blazer said that he needed to think about HOW Sai would be useful. That's the even bigger problem.
Going off of my own playthroughs (oh boy, time to have my entire explanation ignored), I've encountered more than just the paltry number of wyverns you insist are present. This is before Mark joins the ranks. Cavaliers, pegs, and armors are all present en masse, and the only options are tanking with my own cavalier/armor while holding chokepoints, peppering them with mages from across barriers of some kind, and maybe the occaisional lancereaver. Do I have a problem with doing that? I most certainly do not. But yanno, it'd be nice to have an early game axe user that could give me an edge against all these lancers AND have the chance to fully maximize his potential. Asch is good, but being a prepromote limits how much potential he can fully have compared to a T1 axe user. You don't know how relieved I was when Mark finally arrived on the scene, and he was LONG overdue at that point. If someone doesn't go for Asch, for whatever reason, then they're starved of axe mains until Mark's arrival in a game with a lot of lance-based foes.
Who cares if a unit is easily screwed over in level ups? Sai arrives between titans with almost no incentive to use him.