How about different chirality? Different chiral forms would have the same advantages.
Btw. Mirror chiralities are not exactly symmetric. There are tiny asymmetries between chirality due to weak interactions and electron interactions. Weak interactions might affect long hydrocarbon chains. Electrons from beta decay interact with molecules of different centrality with slightly different speeds (difference is something like 0.01%).
Yes, but horses and cows also compete for the same building blocks. And yet, we have horses and cows.
Your argument could be right if there wasn't an abundance of these building blocks.
I think you're just failing to grasp the sheer numbers involved here: there are probably around 10^9 cow-like critters and they interact with other, competing critters every, let's say, 10^4 seconds, with the odds of any one of these interactions being fatal being, again, let's say 1 in 10^4. By comparison, there were probably something like 10^37 (if I counted my orders of magnitude right) biomolecules interacting every 10^-6 seconds where every time some new chemical "A-ha!" moment occurred, that reaction now became 1000x more favorable than any other competing reaction. That's like, what, 41 orders of magnitude; even if I'm off by 10+ orders of magnitude, there was just a whole lot more selective pressure going on during the pre-biotic era. Less favorable chemical pathways were outcompeted on scales so vast you'd need trillions of cow-years to approximate how de-de-de-deselected-for they were.
Nature might be slightly biased due to the asymmetry of the weak force.