So... challenge is like Scotland playing us, right? We don't play well enough, we lose? Sorry Scots, needed a good example. And Scotland is the sort of team you're looking at them challenging? I think saying Celtic countries is a poor pick given how good some of them are, so lets say Scotland and Italy instead. You reckon the USA's growth means they're not far off having a real go at Scotland, is that fair? Not necessarily beating them 50pc of the time, but being a good match to watch and forcing the Scots to play like they mean it? Just trying to define the situation.
Which is... possible. But consider all the investment and tradition Japan have put in. Would you say they're in that position? I probably wouldn't and I think the US is a bit off of being Japan. But fair enough if you do. I do think you're underestimating the difficulties though.
You can only have five on the ice at any given moment, but over the course of the game you'll be extensively using 19 players. Rugby can only have 15 on the pitch at any one moment, but you'll be extensively using 23. The bottlenecks aren't that dissimilar I'd suggest.
It has reached half a million up to u18 based on the stats I saw - .55m to be precise. But yes, a niche, but a very professional one up against mostly small countries. Besides, rugby isn't? Going by one of Heineken's links "More than 115,000 people are registered members of USA Rugby, including over 67,000 high school students." Now, this hockey stat included 6 up, the high school students figure doesn't, but we're still looking at about a 800-900pc growth needed to have the same depth. I know its growing fast and has big potential, but that would be pretty phenomenal growth to achieve and you'd expect that to show at adult level a good 10 years after its achieved. Plus, as noted everywhere, you need quality backing behind those guys.
Now, yeah, this is all shaky stuff, but I'd say its a decent rough comparison of what a decently strong niche American sport looks like on the global stage. And I'd argue that Rugby is a bigger church than hockey at the moment.
Actually you were very specific - "yet Argentina does not have a single professional team, with no hint of this on the horizon"
Its not a professional competition no, but it is the start of steps in that direction. At the very least, I can imagine a second franchise within 10 years. Scotland will try to rise a third Pro Rugby franchise.
A professional domestic league hasn't been a necessity in a lot of countries though, no. Whether that's true in the US with less history I don't know, but I don't know either way. But, in any case, be they amateur or pro, the US will need very strong rugby institutions to talent spot and develop. The US being the US, I think the Colleges gives them a great head start - they're strong institutions, they just need to tie rugby on. But they still need young kids to give scholarships to, and that might take more development time.
Time will tell though. But, as said, I do think you're not weighing the advantages and difficulties fully here.