You're entire argument rests upon the assumption that niobium transitions to the battery industry in a big way. Until it does it hasn't yet, and good luck funding a billion new mine on only the chance that it does. Consider the niobium market sans big battery uptake... yes it has grown to an estimated ~85,000t Nb in 2023, which correlated with possibly the blow-off top in chinese steel demand. A USGS publication had firm Nb production at 75,000t in 2021...
Note the large expansion in capacity by CBMM, which they haven't yet filled with sales yet. Note that "most of Brazil's exports' were to china. That leaves global demand ex-China as not very much, especially with Canada's niobec supplying almost exclusively to Nth America. That leaves very little demand ex-China, ex-Nth America in fact...
You did stumble over one of the reasons there is only room for one new large niobium producer like Luni. Economies of scale... the financial law of diminishing cost of production and internal rate of return with increasing production. Luni will need to start large to make a sensible return on very large capex, then the beancounters will be pushing for stage 2, stage 3 etc to maximise profits on such a large resource in high cost Arunta from scratch. Any other new niobium player will also be carrying large capex and debt burden while competing with CBMM, who can expand existing operations in low-cost Brazil if required so much cheaper than greenfields (to say otherwise is ridiculous).
By all means, hang your hat on niobium winning the Cambrian Explosion of new battery lifeforms to dominate the future. I'd rather wait to see who wins, or if niobium is actually substituted out of even steel production from the explosion in materials science development driven by AI computing speed. I politely suggest that bankers are conservative types, also likely to wait and see how the niobium market absorbs luni and battery demand pans out before backing in a potentially fatal oversupply price war scenario...
Time will tell... GLTAH
(20min delay)