I assume you're an audiophile, no?
I think this is an "Apple Jacks commercial" situation. As an audiophile, you're all like, "Why do you listen to this when it's clearly not good synth?" ("Why do you eat Apple Jacks when it doesn't taste like apples?") -- to which we respond, "Because it sounds good!" ("We eat what we like!")
I've had this argument before (when defending Ys I & II Complete's OST as being superior to Ys I & II Chronicles' in many instances, despite LITERALLY being nothing more than MIDI), and in short, one of the ways in which Falcom Sound Team is amazing to me is that they can take "cheap" synth that audiophiles generally dismiss, and do AMAZING things with it. They breathe new life into old, tired sounds, just as they breathe new life into old, tired cliches... and that's part of their charm for me, I think.
It's like I wrote in an old review, "Falcom doesn't reinvent the wheel, they just make it rounder." They take existing things and perfect them. This new Ys IV tells a story that involves amnesia, but you know what? I'm not worried. I know Falcom will be able to make me believe the old, tired cliche of an amnesiac hero, and even CARE about it. And the new Ys IV may also feature some music composed with "outdated" modularized synth... but you know what? I *miss* that. Fancy, modern synth and live instrumentation is all well and good, but there's a certain charm to "slumming it"... especially when it's done well. And every time Falcom works with old synth -- even synth that audiophiles deem virtually worthless -- they always create masterpieces with it. Masterpieces that simply wouldn't have as much impact if they were performed in any other way.
...Though I'm just making that argument in general. In this particular case, I don't get your point at all. The synth used in that Ys IV trailer sounds AMAZING to me. It doesn't sound low-quality in the slightest, and certainly sounds NOTHING like that "cheap MIDI" you linked. So yeah... I don't get it, man. Sorry.