Economic dividends don't buy tickets. We seem to be talking about 2 different things, I'm talking about system operating, construction, and ongoing capital costs and you're talking about larger societal benefits including potential increased non-railroad economic activity or less "carbon emissions". For example NYC's MTA loses about $6b/year and that's an entirely separate consideration from whether NYC as a whole benefits economically.
https://medium.com/@johnnyknocke/th...-dollars-a-year-and-nobody-cares-d0d23093b2d8
You don't look at it in a vacuum, you compare the opportunity costs of HSR versus other options. Perhaps building additional airports creates just as much economic benefit at half the construction costs and runs at an operating profit whereas HSR would have operating losses of $5b annually (I'm obviously making up numbers). In that scenario it's a very valid question whether there actually is a business case for HSR. Just having the ability to capture some non-zero market share isn't a business case, it has to offer at least some USF (unique selling feature) over competitors and if it's not cost then something else has to be in play (saying "I think trains are a more sophisticated way to travel than plane" isn't really a business case).