Because the same criterion that Brainstorm is banned under, is the same circumstance that Ponder (and likely Preordain) will likely then find themselves in. Because people will always just run the "next best thing." This sort of ban-rubric is a race to the bottom.
If you take the position that, no, the banning of Brainstorm is a special case, you have to justify this case. Why is the Brainstorm case special? Also, since it is plausible that Brainstorm is perfectly fine without the presence of fetch lands, explain why fetch lands wouldn't be the ones banned, especially since cards like Deathrite are only, presumably, good with fetch lands as well. Now you need to articulate why fetch lands are alright and Brainstorm is not. Then, once you have done that, you go right back and look at the resultant meta. What if Tundra has a 75% meta share? Are duals bannable? Why or why not? What about Force of Will?
The issue is, I see no way to articulate a fully spelled out manner and fashion of what "normative" Legacy is. I see no way for there to be "one rubric" of what can and should be banned. So there is no way, in my opinion, to have some cut and dry quantitative meta-threshold, or something else, to define what action should be taken in Legacy.
Legacy just
is 4x Brainstorm, by fiat. You can hate it. You can claim that is shouldn't be. But it just is the case. So, you have the option of playing it, fighting against it, or just not playing. You can say none of those are "fair" options and that might well be the case. But the fact of it is, that is the case. Now, you might think I am spuriously making a sort of Is/Ought claim, that because Legacy
is the Brainstorm format, it
ought to be the Brainstorm format. This is not my point, or my position. I have no idea what Legacy
ought to be. The only thing I can possibly do is say what I think Legacy
ought not to be.
I even tried making a thread to get an idea of what people thought defined/should define Legacy years ago. I don't dwell in a place of hypothetical card-pools. There is a near infinite amount of hypothetical card-pools and I'm not interested in any of them, really.
In our play group, we have a running joke of saying "just play your cards." It applies to a lot of things, but works here as well. Just play your cards. Is Brainstorm oppressive? Maybe. Then just play it. Or, fight it. If you say, "well, I shouldn't
have to" well, I'm sorry, but if you want to play nominal Legacy, you do. But no one
makes you play Legacy. If you don't like it, don't play it. If you think some other card pool is "better," make that case. Get other people on board and play
that format. But the idea that "Optimal Legacy," like Shangri-La is just out there, waiting for someone to discover this utopian ideal card-pool where every card is rationally justified in it's existence, where no card could unjustly impose itself on any others, where anything and everything is viable and everyone gets a free lunch. But actual Legacy, the Legacy we have and are likely to
always have to varying degrees is actually a intuitionally derived hell-hole, where the only way you stay alive is by taking life from something else, where caprice and whim define existence and every moment is a struggle to survive.
If you want utopia Legacy, then go on and show us how it can be done. Not by some hypothetical rational exercise, but with actual empirical results. Then we might see that this hypothetical card pool really is the promised "ideal" one. Until then, no, "we" are not likely to accept the notion that "Optimal Legacy" is only a ban away.
So, why is Lurrus ban-worthy where Brainstorm is not? I don't know. But I agree with it. Whether it is a notion of historicism, or of perception, or whatever, once again, you will never get to "Optimal Legacy" with a fully-rationalized ban-rubric. But, then again, maybe I am wrong. If so, I'd invite anyone to come up with a fully-rational, articulated, quantitative
and qualitative manner to conclusively display what is "Optimal Legacy" and what is expressly
not.
In the mean time, I'll just play my cards.