We have been seeing the first 32 Bit boards getting around more and more over the last year or so, using 8 bit as some sort of a argument or point is what exactly? 8 bit apart from the bottom end of the market will be gone fairly soon IMO.
Well, you were talking about RAMPS. RAMPs is pretty much equivalent with 8bit AVRs, the original RAMPS was a shield for Arduino. That evolved over time into integrated all-in-one boards but the architecture remained pretty much the same.
Marlin is a fairly well developed and constantly evolving standalone interface to a CNC machine in the majority of cases it is 3D printers. There is no reason I can think of it couldn't be forked for example to a non ramps cnc router board in particular if done from scratch.
Marlin itself is not much of an issue, I believe there are some forks/variants for STM32 chips and others. It is more the RAMPs/driver part of the equation which sucks if you try to reuse a common 3D printer board with a CNC machine.
At the Hobby end sizes (lets just pick 3020 and smaller) what is needed is a some modification toward a 3D printer standalone solution. Local Data storage, G Code interpretation, Interface for jogging, setup etc. You are making an assumption that the CNC board would use bottom end Chinese step stick drivers like the 5+ year old RAMPS boards to do this would be a really poor choice given what hardware is already in use and available now.
Again, it was you talking about RAMPS and those are almost all designed around this. If you didn't mean that, then sorry but I don't read minds yet.
BTW, "Stepstick" is not Chinese, that's Pololu 's invention - a small DIP carrier board with an Allegro A4988 stepper driver (or similar - e.g. the Trinamic ones are popular) that gets inserted into female headers on the main board. When talking about "Stepstick" it is more about this form factor than anything else today. The idea behind this was to make the drivers easy to replace because the early designs they were failing regularly due to various overloading and short circuits on poorly built RepRaps (where this design comes from - RAMPS stands for "RepRap Arduino Mega Pololu Shield")
And even if the board doesn't use these "plug-in" driver boards, they often use the same stepper driver chips directly on the PCB. Either way they would be very underpowered/lacking cooling for a CNC machine.
OTOH, if you want to upgrade a router such as 3020, it shouldn't be too hard to interface a board running Marlin with the original electronics if it is serviceable (some are reasonably built but there are also some very poor ones around). It would need only a custom cable, removing the stepper drivers from the Marlin board (if it has any) and maybe some minor code modifications - the control interface for the "Stepstick" style stepper drivers is very similar (if not identical) to what these cheap routers use (a direction signal and a pulse for each step). That would be, IMO, better than trying to rip the old electronics out and trying to use a 3D printer board there instead.