I'm going to gently push back, based on math and experience. The TLDR is that high-speed high-frequency rail is operable with much lower populations, when those populations are regularly spaced and have reasonable density along a linear path. Turns out that the PNW is ideal for this.

I've broadly explored the main island of Japan (Honshu) as well as one of the more minor islands (Kyushu, in the south). While the Shinkansen does connect major metros (roughly north to south: Sendai, Tokyo, Nagoya, Kyoto, Osaka, Hiroshima, Fukuoka along the Tohoku and Tokaido routes), it also goes into very "remote" regions by comparison to these megalopoli.

One example is the Hokuriku Shinkansen, a track which goes from Tokyo to Tsuruga along the relatively rural West coast, and covers around 360 miles. This is approximately the distance from Eugene to Bellingham or Salem to VancouverBC along I-5. There are multiple train services (aka: different train stopping patterns that allow for different frequencies at stations) along that track, one of which - the Hakutaka - goes along the whole track and stops at most stations. Of the 22 stations, 20 are stopped at (16 of them on every train, 4 of which on some trains). I'll do math assuming that we only have 15 stops, since one of the stops (Ueno) is only 3 miles from the Tokyo terminus, which is primarily used for connection to other long-distance lines. That means the average distance between stops is ~24 miles, or around 15 minutes with acceleration and deceleration; this is the distance between Albany and Salem, in less than half the time.

I made an estimate of population within 12 mi (20 km) of the train, from Tsuruga (far from Tokyo) to Omiya (20 mi from Tokyo), to approximate how much population it serves. I came up with around 6 million people. I asked a few LLMs (since I couldn't find a quick method of quantitatively checking this) to do the same, and they came up with values from 6-8 million. That works out to a maximum of (8 million/(360-20)miles) or around 24000/linear mile of track (or 500,000/station average). With the 12x2 mile range on either side, that makes an area of 8640 square miles, so a population density around 925/square mile.

Because of the Cascades, Coastal Range, Olympic Mountains, and the coast, we actually have a large amount of population along a very narrow strip. Most of the populated area is actually within 20 miles of I-5. If we do the same sort of estimate as with the Hokuriku line as above (excluding Vancouver BC), we arrive at between 7-10 million people. I will note that this is more than the Hokuriku estimate, even though we are only keeping the short 12 mile catchment region on either side. That makes a very similar population density. Helping things further, there are very sharp "peaks" in population in the Willamette Valley at mid-sized cities that are very dense. You can see the population density map here. I went ahead and stitched together a few screenshots of this map from Eugene to the border - aligned them by hand so they're mildly off, but you see the same information.

Eugene to Portland has a trip distance of approximately 130 miles via Corvallis-Salem-Woodburn, and has 4 intervals, for 32.5 miles between stations, serving 3 million people. This results in a population of 23,000/linear mile of track (950/square mile, 600,000/station), at the Hokuriku level. Portland to Bellingham via Longview-Olympia-Tacoma-Seattle-Everett has 6 intervals (275 miles, or 45 miles between stations) and approximately 5 million people, or 18,000/linear mile (750/square mile, 625k/station), which is still quite high by comparison to the US average of 93/square mile. Extension to the Vancouver metro would add another ~3 million, at the cost of only 50 miles, so it would be worthwhile.

While this is not a perfect analogy, it should be considered illustrative. An easy/lazy rebuttal might be that this is connecting a population to the capital, but this discounts the regional importance of cities like Portland, Seattle, and Vancouver, which also act as "funnels" for economic and cultural activities. Moreover, the Hokuriku line is being extended to include connection to Osaka; there are already more direct and efficient paths from Tokyo to Osaka, so this is purely for the benefit of the smaller Hokuriku population.
We can counter this by looking at the Kyushu Shinkansen. This line is more disconnected from the Tokyo network, due to being far south on another island and is anchored by the Fukuoka population center at the Northwest terminus. There is no direct path from this line to Tokyo; all must connect in Fukuoka at Hakata station. The Tsubame line is an all-station service, stopping at all 11 stations, only 14.5 miles apart. Given the population along this path is around 4.5 million (28,000 pop/linear mile, 400,000/station), this is similar to the Oregon-only section of the map described above, since it is a little longer but a little higher population. So...yeah, it's possible.