Man, really, I'll elaborate on my opinion.
In a certain sense, you are about right. Yea, I do admit that
in the best case scenario, it's fine if things happen how you want them to. Okay, let him have character development, or whatever.
As for my own opinion, I consider LNs, manga and anime a form of entertainment. And I do believe, if I myself may say so, that most of the people who complain about your point of wiew are talking about something I'm going to lay out from now.
For me, Izayoi's character is fun. Entertaining to the point that in the anime version, I've watched the Algol battle at least 20 times, and the point where he yells "Bitch please!" when kicking the Gift to petrify the world apart (fffansubs english sub) is probably about 200 times on my watchlist. That's because his character for me is highly unusual. I don't particularly have a narrow interest in anime goods (I prefer romantic comedy to some degree, and totally despise any form of tragedy and drama, but that's about it), and I'm nearing 800 series watched, without discrimination in either age or story. And I virtually never ran across such a foul-mouthed, self-centered asshole (well, the only one I can think of off the top of my hat would be Kazuma in Kaze no Stigma, but then again, he didn't have the same "reckless youthful abandon" Izayoi has). For me, he's a fresh new aspect on the main characters.
Reason for that is simply - there are certain "skeletons" (and not that many of them at all) that you can identify as a basis for many kinds of entertainment in this field of work (be it manga, LN, anime, Drama CD, etc etc). To put it simply - they're mostly easy to foresee and calculate. Most of the time, I easily foresee a certain development, to the point where I'd wager on it without a second thought (and believe it or not, I'm mostly right). This one though, is a fresh thing for me, since although it's easy to calculate, it just
plays itself out in such a fun way. Make no mistake - I'm not againtst something developing slowly, or not developing at all, but I value my entertainment above all, and I'm not asking for classy english literature when I'm excersising my consumer rights
. I just want to have fun, and for me, this series is fun with Izayoi's "high and mighty" attitude. That's why I don't want to change him - I want to see him breaking through every problem with his asshat style. Because for me, it's simply more fun that way.
Not every story needs "depth" imho. In my country (don't want general hate, so I won't name it) 200x's best comedy anime through a vote turned out to be Lucky Star. What the hell is that entire series about? Nothing. Not virtually, but actually nothing. And still it's just damn fun, like instead of choosing between constructive and fun pastimes, you simply goof around and laze, and still make effective use of your own "fun time" (Rosario to Vampire being the worst series though, I'd love to kill someone for that blunder ...). If we go strictly by entertainment, I think things are fine the way they are.
Then again, with the current developments with Azi Dahaka (who's actually a minor demon in eastern mythology, so he really must be considered some underling fodder in this LN too), it's more than likely to turn out into something you're petitioning for, at least to some degree.