Corrin wasn't even very well known within FE community.
His game wasn't even available outside of Japan.
I think you're highly underestimating the chances of those characters tbh.
You do have to remember that FE Fates was a promotional game and still out in Japan, so Corrin was still recognized and had something to advertise.
Characters like Geno, Isaac, etc. don't have anything to promote or offer. Nor are they veterans or highly recognizable, which may diminish their chances even more.
I more or less agree with the point, as I doubt we'll see obscure and old characters make it into DLC, as there probably isn't much profit to make off the characters alone.
Unless they have a season pass model this time around, I'm honestly not expecting to see too many obscure or niche characters makes it as DLC.
Would I love to? Yeah.
I just personally doubt it.