Opinel is kind of the embodiment of elegant simplicity, economy and value for the money. They're easy to manufacture, easy to maintain and easy to resharpen when they need it, using simple sharpening media. They've got a good thing going and they're selling by the millions worldwide, with the recipe they've already got. It'd be difficult for them to keep that up if they had to retool for specialty steels, which would be much more expensive to grind and work into shape.
The cost of the raw steel isn't so much the issue, but increased costs of stamping out and grinding wear-resistant tool steel blades is the bigger factor. It's the main reason Buck Knives transitioned away from using 440C back in the day, to something that was easier (on the tooling) to stamp out and grind in a cost-efficient manner; that was 425M, then 420HC later on. [ Edited to add: It turns out now, the traditional maker GEC (Great Eastern Cutlery) is also apparently considering something less wear-resistant than 440C in it's stainless offerings, for precisely the same reason. ]
I wouldn't knock any knife/steel enthusiast for doing the customizing on their own, with a custom blade of their own choice. But there's little to be gained by Opinel themselves, in trying to fix or improve something that's time-proven and really doesn't need fixing, for the sake of what would be a niche-market enterprise.
There's always a chance Opinel, someday, could start a 'Custom Shop' operation like Buck did, offering special handle materials and premium steel options. But I'm certain those knives would be sold at prices a few multiples higher than what they currently sell for about $15 apiece, in their No. 08 size.
