Speaking to this particular claim, loads of people have had the chance to mission "My Immortal" over the years, and no one's done it. If Iximaz comes along and actually wants to, and is pretty sure she can pull it off (which I think we all know by now that she can), why shouldn't she? How long has this particular collectible lightning-shooting red car been sitting on the lot, gathering dust?
Same argument goes for the other Legendaries Ix has taken. Most of them, if not all of them, have been sitting around unmissioned for years, waiting for someone with the right combination of interest, time, and perseverance to come along and claim them. It seems like Ix is just the rare individual they've been waiting for. I don't think assuming that another like her is going to come along anytime soon makes sense.
That said, I do take the point that putting so many Legendaries in one spin-off seems... eh, I don't know, exactly. Vaguely wrong somehow. Maybe because it seems to clash with how things appear to work in-universe? But we're not talking about in-universe, we're talking about a spin-off. But then, supposing we're looking for some sort of normal missions-to-Legendaries ratio, the Aviator has, I think, something like 40 completed missions and interludes to her name. Four of those are Legendaries. That's, what, a whopping ten percent of the spin-off?
To compare, Dafydd's spin-off totals 18 stories, two of which are Legendaries. (Three, if you count "Woodsprite of the North," but we won't here.) His ratio is actually a little higher than Ave's one in ten. The only difference between them is that his ended and hers is still going, so there's no longer a possibility that he'll do more.
... I dunno, guys. The truth is, I don't really have a huge stake in this either way. Personally, I'm only interested in one Legendary, and I don't much care what happens to the rest. Plus, I consider myself to have tackled one already in "Blood Raining Night"; it's just that it isn't acknowledged as such because it's freaking impossible to get this community to agree on a policy for deciding when to acknowledge new ones, so it doesn't count. Supposing you wanted to hunt Legendary-quality badfic without sparking a debate like this one, all you'd have to do is keep an eye out in other places on the Internet and nab the ones the PPC hasn't tagged yet. So, I guess I don't really see the point of getting fussed about it.
~Neshomeh