I reckon this underestimates the value of JL1, where those businesses had invested millions over many years in building a critical mass of full time players. Depending on your timeframe, well in excess of that $250m (in Yen) had been put into the Japanese rugby ecosystem
MLR has some resources, but it's nowehere near JL1. The USA don't have that mass of fully pro players to build on
Yeah it's not like for like. Japan benefits a lot from the higher level of player that becomes eligible for them through residency, because of their pro league. The MLR attracts 2nd tier players in comparison (though these can still be very useful players that help improve the player pool).
But the US has it's own advantages compared to Japan, even when only considering what could be done in the short period of time leading up to 2031 - and without needing to invest hugely into the domestic ecosystem. It would be interesting to know how many there are, but I'd be confident there are a lot more professional (or strong under 20's) players in Australia/NZ/France/England/South Africa etc that are US eligible, than there are that are Japan eligible.
My thinking is imagine World Rugby/USA Rugby had a ~US$10 million budget per year to run a full time US Eagles eligible squad from 2026-2031, with a tier 1 level coaching staff. How good do you think that team could be by 2031? (noting they might still have some star players they could pick from European club rugby etc).