The factual basis of your assertion is absolutely true, but your attitude is unhelpful and defeatist.

There is a chasm between "a state actor throws an 0day at you" and "Google remotely installs an app on your phone". The latter is done at scale. The former is expensive, risky, and used relatively rarely.

If you're organizing a protest movement, it's totally reasonable to factor government 0days into your threat model. For more boring people, running GrapheneOS is a great way to reduce the attack surface they expose to the advertising and mass surveillance industrial complex.

its not exactly a 0day if the ISP is communicating (through an intermediary) to a card the ISP gave you, that's just normal, unexpensive

And this is like, literally a state actor installing an app in this case?

In this case it requires the presence of Google Play Services. I'm unsure if there's any evidence that they can install apps without it being present.

It's hard to get good info on what capabilities it does have. Here's what I've gathered though I'd like to learn more:

Modems are often isolated by being connected via USB, or if on your SoC the modem has DMA then it's isolated via IOMMU groups.

SIM cards have to implement the E911 feature which allows 911 operators to toggle a cell phone into "stay online no matter what" mode.

Some SIM cards have additional apps installed on them, which allows attacks like SIMjacker and WIBattack.

Two datapoints:

1) http://ramtin-amin.fr/#nvmepcie, http://ramtin-amin.fr/#nvmedma (the two articles are separate but the first provides incidental context for the second) the iPhone 6 kinda maybe sorta didn't dot the Is and cross the Ts with the MMU side of things. So, USB is awesome in that the failure state is "probably can't RCE".

2) I read a comment on here, which I should be able to re-find, but hn.algolia is not cooperating, suggesting that the system design of a particular AGPS implementation (a few years ago) interposed the GPS in between the CPU and the cellular radio such that the GPS SoC could do HTTP requests to grab its almanac that all of Android, down to the kernel, had no idea about.

IMHO this level of security paranoia is at the end of the day a micro-optimization. For any given device, you're looking at maybe two or three dozen Things Containing ALUs™ (often buried inside subcomponents buried inside other things); one or two concentrations of several billion transistors; and an unknown proportion of manglement, incompetence, cost-cutting, internal compromise (because guarantee there's none), and Agreements™. Honestly: give up, and declare that whatever makes you feel better is enough.

Do any of the privacy oriented custom ROMs protect against that? I can't imagine their maintainers seeing code that just installs any app the ISP wants and be okay with it.

The problem is, its usually cheaper the more things you can shove into the 1 hardware item, so you have your cellular hardware in the same chip as your CPU and GPU. Not much a ROM can do about this unless the chip itself supports disabling direct memory across the two items, + does it correctly, + doesn't allow it to be reversed from the other side, + you would also need the datasheet to find out how to implement this.

Generally why privacy roms don't support more than 1 or 2 brands total, I guess.

There are also platforms with strict division between the seperate parts of hardware, la pinephone and the librem5

Smartphones are usable without sim cards.