If your goal is to reduce your attack surface, then yes, even a modem without a SIM is too much.
The modem in your phone isn't like a modem from the days of dialup, it's more like a cable modem. More often than not, the modem is its own entire microprocessor, ram, i/o, etc and then communicates with the device's CPU over a mixture of serial, i2c, spi, or other busses. For instance, in my Pinephone, the modem is a Qualcomm MDM9607, which is a single core arm cpu that has 256mb of ram and 256mb of nand on its package, it literally runs its own entire operating system (linux in its case) separate from what the CPU of the phone does.
This CPU can also have its own connection to the battery, which is how, for example, iPhones can remain 'findable' even when the phone's CPU is otherwise powered off and at rest. The modem sips at the little remaining power in the battery to power itself and the GPS chip to report the devices location.
As for 'removing the sim' that doesn't prevent the device from connecting to a network, just authenticating with it, typically. Your sim card is just a standard identifier and a little bit of storage that the modem can read and write to for things like storing contacts and SMS messages. All of which can be done in software as well (known as an eSIM these days).
Edit: Here's a link to the wiki page on the Pinephone's modem, just to give you an idea of what a cell phone's modem can be capable of, and keep in mind, it uses a rather old, outdated, and unpopular modem, other modems may have more features: https://wiki.pine64.org/wiki/PineModems
I don't think it's equivalent judging by the fact that you can still make emergency calls, which means it's able to talk to the network anyway. I remember than an older Samsung I had (GS3 IIRC) would show the signal level even withou a SIM installed.
Don't know what happens when you put it in airplane mode, though.
The modem in your phone isn't like a modem from the days of dialup, it's more like a cable modem. More often than not, the modem is its own entire microprocessor, ram, i/o, etc and then communicates with the device's CPU over a mixture of serial, i2c, spi, or other busses. For instance, in my Pinephone, the modem is a Qualcomm MDM9607, which is a single core arm cpu that has 256mb of ram and 256mb of nand on its package, it literally runs its own entire operating system (linux in its case) separate from what the CPU of the phone does.
This CPU can also have its own connection to the battery, which is how, for example, iPhones can remain 'findable' even when the phone's CPU is otherwise powered off and at rest. The modem sips at the little remaining power in the battery to power itself and the GPS chip to report the devices location.
As for 'removing the sim' that doesn't prevent the device from connecting to a network, just authenticating with it, typically. Your sim card is just a standard identifier and a little bit of storage that the modem can read and write to for things like storing contacts and SMS messages. All of which can be done in software as well (known as an eSIM these days).
Edit: Here's a link to the wiki page on the Pinephone's modem, just to give you an idea of what a cell phone's modem can be capable of, and keep in mind, it uses a rather old, outdated, and unpopular modem, other modems may have more features: https://wiki.pine64.org/wiki/PineModems