Keeping something launched into LEO 1998 in operation for 26+ years seems to be pushing it. Many parts have been replaced over time, but the shell is in a very harsh environment. Hopefully with the modular nature they should be able to let old sections go.
My worry here is that if ISS gets shut down completely without another station being already at least partially in orbit, there won't be enough political will to start another project like that.
Historically, that's probably the most likely outcome. NASA has always faced tough political battles for funding all the way back to the middle of the Apollo program and nowhere has that been more apparent than when NASA was trying to replace one major project with another. If you're interested, John Logsdon's After Apollo? Richard Nixon and the American Space Program is a fascinating--albeit depressing--book on the politics of NASA funding. It mainly focuses on Apollo and the transition to STS (and covers some of the alternative paths people at NASA wanted to pursue), but I have no doubt whatsoever that many of the same challenges will plague any ISS replacement, whatever it may be.
That would be an ideal outcome. The ISS is a huge resource sink compared to the kinds of unmanned probes that could be launched at a fraction of its budget.
The problem with the International Space Station (ISS) is the ISS. It was an exercise in 80s and 90s design, and you could launch an equally capable science platform at a lower cost than refurbishing the current station.
One of the main reasons why NASA is reluctant to replace the existing station is that they are afraid that the program would get cancelled, and they will be left with no station.
Also the I part of ISS kept the world space research orbiting around NASA. For example Brazil make every single research that requires a station (including vehicles, modules and medical science) use the ISS.
Without the ISS Brazil could for example attempt its own station, or help China instead.
NASA is very powerful, but if everyone help China or Russia instead, it would be bad for USA strategically.
As a NASA employee, I find the phrase "NASA is very powerful" amussing in any context. That said, I think the far more likely outcome of the ISS being abandoned without a replacement would be that most countries would simply not invest at all in space-based research. The Russians just don't have the budget to build a station on their own, and I doubt the Chinese would welcome international collaboration.
I am not sure that many countries would want to get involved in the Chinese space program, which has always been secretive, and dominated by nationalist goals rather than scientific ones. It seems that the Russians would welcome other participants in their program, though it seems unlikely that Brazil would want to fund what appears to be a struggling and lackluster Russian program.
I am also not sure why a reduction in international collaborators for NASA would be strategically bad for the USA. I am a strong proponent of space exploration, but NASA is not an important part of US military strategy; NASA is mostly a luxury which allows the US to demonstrate its technological supremacy.
"Control" of space isn't a big deal as long as nations respect the no weapons treaty. Afaik it is possible for ground-based weapons to destroy satellites too. I guess a satellite that could find submarines or hidden nuke sites would be useful in peacetime, if you were planning a first strike.
