That's the big one. What HSR needs is not even higher top speed but some control breakthrough that allows much higher train frequencies in order to cut down on stops via more trains that can serve more specialized connections (e.g. the local/through etc). No idea what that breakthrough might be, perhaps something as crazy as rocket-assisted emergency braking.
If you go from Shanghai to Suzhou via HSR a train goes roughly every 15 minutes. Most trains going east go through Suzhou (since it is a major hub) and it is roughly 30 minutes away which is a decent distance so worth stopping for. The train only gets to maximum speed for about 15 minutes in the middle.
At 1000 or even just 600 km/h, wind resistance would do some damage just by letting off the accelerator.
This would be really fun to watch in action.
State-of-the-art high-speed trains at 350 km/h tend to waste less time than planes for distances shorter than about 1000 km if you include the time it takes to get to the airport, check-in, go through security, and all that nonsense. Beyond 1000 km journeys, planes tend to be more time-saving.
1000 km/h is about the cruising speed of an aircraft so if it were possible to achieve that with a train, it would no longer make sense to take planes for the same journey -- which would be HUGE plus for comfort for, say, trips between Europe and Asia. Imagine for a 10-hour journey that you could walk around freely, enjoy massive amounts of legroom, enjoy the views of Siberia, sleep in peace and quiet, dine at a dining table, and not have to be strapped into a seat all the time. For high-volume routes it is also likely to be lower-carbon in the long run than air travel, and at the very least, can be powered on something more sustainable and less polluting than jet fuel. The bulk of your environmental impact would come from laying the track itself, so it would have to be a route of extremely high tourism or business importance, e.g. Shenzhen-Shanghai-Beijing-Moscow-Warsaw-Frankfurt-Paris-London or some such. Or Los Angeles to New York.
Not to mention the one critical advantage of the train, run out of fuel or some breakdown, you just stop in the middle of nowhere, on a plane, it's likely a different ending unless flying near Hudson river with an amazing pilot.
China's high speed rail system has had 1 accident with 40 deaths out of several billion passenger kilometers of trips, and that was involving a slower, older technology. The latest, fastest trains in China have not had any accidents to date.
Japan's Shinkansen has had 2 derailments in half a century of operation, one due to an earthquake and one due to inclement weather. Both incidents had zero injuries.
The European systems have had notably higher accident rates but still surpass the safety records of planes.
I have used the high speed line between Wuhan and GZ (and through to HK) several times. It’s currently a 4.5 hour direct journey (once per day) or a 5 hour journey if you change at GZ or Shenzhen. The HSR is 300ish km/h at the moment and that route stops about 4-5 times along the way, off memory.
Aside, I also don’t know how much benefit 1000 km/h travel would bring. That’s an enormous energy requirement to accelerate up to such a speed only to stop 4-5 times along the way. 600 km/h would likely be much more realistic, because if you shorten the duration too much you run into the wall of needing to accel/decel constantly to make the stops.
Lastly, the HSR there is already capable of 350 km/h at least, but they limited it to 300 km/h after a massive derailment that caused a heap of fatalities years ago and never increased it back up again. It may be reading into it too much, but it may also be telling that they didn’t see the benefit of upping the top speed again even when considering the loss of face.