That is absolutely not an MCU class footprint. Anything with an "M" when talking about memory isn't really an MCU. For evidence I cite the ST page on all their micros: https://www.st.com/en/microcontrollers-microprocessors/stm32...
Only the very very high performance ones are >1MB of RAM.
(And I would argue that the ESP32 is an MCU even in this configuration — mostly because it satisfies ultra-low-power-on-idle requirements that most people expect for "pick up, use, put down, holds a charge until you pick it up again" devices.)
So, sure, if you mean the kind of $0.07 MCU IC you'd stuff in a keyboard or mouse, that's definitely not going to be running Nerves (or any other kind of dynamic runtime. You need to go full bare-metal on these.)
But if you mean the kind of $2–$8 MCU IC you'd stuff in a webcam, or a managed switch, or a battery-powered soldering iron, or a stick vacuum cleaner with auto suction-level detection, or a kitchen range/microwave/etc with soft-touch controls and an LCD — use-cases where there's more-than-enough profit margin to go around — then yeah, those'll run Nerves just fine.
If you ship MCUs with 16MB of RAM routinely, you're either working with graphics or are actually insane.
A managed switch is very computy heavy, and does usually run a microprocessor with a full RTOS, if not Linux itself, which probably costs in the mult-dollar range. It's also not something most people have at home, outside of the switch built into their router. Everything elese you mentioned usually runs on microcontrollers with under 1 MiB of RAM. For example, Infineon's CYUSB306X series ASICs for webcams come in two RAM sizes: 256 KiB or 512 KiB, despite handling gigabits per second of data, and having an MCU at all isn't even necessary. (https://www.latticesemi.com/usb3#_CDED461DE15245C78A2973D4A4...) The Pinecil's Bouffalo BL706 MCU has 123 KiB of RAM, despite being a low-volume product where design time matters more than component cost. (https://wiki.pine64.org/wiki/Pinecil, https://en.bouffalolab.com/product/?type=detail&id=8) Microwave ovens are so high volume that they often don't even use packaged microcontrollers, mounting the die directly on the PCB, with an epoxy blob protecting it, and there's no way any would splurge on megabytes of SRAM. The most advanced microwave oven I've seen was from the 90's and definitely didn't splurge on an microprocessor. (https://www.youtube.com/watch?v=UiS27feX8o0)
A slow MCU with external memory could run anything, like booting into linux on an AVR (https://www.avrfreaks.net/s/topic/a5C3l000000BrFREA0/t392375) but it's going to be extremely slow and not practical for any commercial product, which if produced in any volume will have as little RAM as possible.
The Sub <$1 you refer do will have <100k of these. (STM32C0, G0 etc)
ST is actually moving away from the 2Mb MCUs, and instead offering ~1Mb with Octospi. I believe the intent is to use offboard flash if you want more. (e.g. the newer H7 variants)