I don't intend to get rid of my smartphone, but I do carry a larger device with me, and try to use the phone increasingly as just a dumbphone and cell modem for that device to tether to.

That may not be viable for everyone --- it's not a great solution to "I'm standing in line and want to use a small device one-handed". And iOS/Android smartphones are heavily optimized to use very little power, and any other devices mean more power. It probably means carrying a larger case/bag/backpack of some sort with you. And most phone software is designed to know about and be aware of cell network constraints, like acting differently based on whether you're connected to a cell network for data or a WiFi network for data.

However, it doesn't require shifting to a new phone ecosystem. It also makes any such future transition easier --- if I have a lot of experience tied up in Android/iOS smartphone software, then there's a fair bit of lock-in, since shifting to another platform means throwing out a lot of experience in that phone software. If my phone is just a dumbphone and a cell modem, then it's pretty easy to switch.

And it's got some other pleasant perks. Phone OSes tend to be relatively-limited environments. They're fine for content consumption, like watching YouTube or something, but they're considerably less-capable in a wide range of software areas than desktop OSes. A smartphone has limited cooling; laptops are significantly more-able to deal with heat. Due to very limited physical space, smartphones usually have very few external connectors --- you probably get only a single USB-C connector, and no on-phone headphones jack. You're probably looking at a USB hub or adapters and rigging up pass-through power if you want anything else. Laptops normally have a variety of USB connectors, a headphones jack, maybe a wired Ethernet connector, maybe an external display jack. Laptops tend to have a larger battery, so it's reasonable to use the laptop to power external devices like trackballs/larger trackpads, keyboards, etc. You get a larger display, so you don't have to deal with the workarounds that smartphones have to do to make their small screens as usable as possible. You don't have to deal with the space constraints that make a touchscreen necessary, having your fingers in front of whatever you're looking at (though you can get larger devices that do have touchscreens, if you want). You have far more choices on hardware, and that hardware is more-customizable (in part because the hardware likely isn't an SoC, though you can get an SoC-based laptop if you want). Software support isn't a smartphone-style "N years, tied to the phone hardware vendor, at which point you either use insecure software or throw the phone out and buy a new one".