Let's look at the lowest end chip in the discussion. Almost certainly the SAM9x60.... it is a $5 ARMv5 MMU chip supporting DDR2/LPDDR/DDR3/LPDDR3/PSRAM, a variety of embedded RAM and 'old desktop RAM' and mobile RAM.
Yes it's 32-bit but at 600MHz and GBits of RAM support. But you can seriously mass produce a computer under $10 with the chip (so long as you support 4-layer PCBs that can breakout the 0.75mm pitch BGA). As in, the reference design with DDR2 RAM is a 4-layer design.
There are a few Rockchips and such that are (rather large) TQFP that are arguably easier. But since DDR RAM is BGA I think it's safe to assume BGA level PCB layout as a point of simplicity.
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Everything smaller than this category of 32-bit / ARMv5 chips (be it Microchip SAM9x60, or competing Rockchips or AllWinner) is a microcontroller wholly unsuitable for running Linux as we know it.
If you cannot reach 64MBs of RAM, Linux is simply unusable. Even for embedded purposes. You really should be using like FreeRTOS or something else at that point.
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Linux drawing the line at 64MB hardware built within the last 20 years is.... reasonable? Maybe too reasonable. I mean I love the fact that the SAM9x60 is still usable for modern and new designs but somewhere you have to draw the line.
ARMv5 is too old to compile even like Node.js. I'm serious when I say this stuff is old. It's an environment already alien to typical Linux users.