Why wouldn't a faster machine with more RAM be a multitasking machine? (Obviously without extras you are limited regarding security etc, but plenty early multitasking machines didn't have that)
It's easy to write a scheduler for a 6502 as there's so little to save, though you'll need to be very careful about stack usage, and you might do better with a specialised scheduler (e.g. for C64 BASIC) as a lot of code you might want to run may store additional state in fixed locations.
The Game Boy Color has a 16-bit address space and almost all its games are 1MB or larger (although that's ROM rather than RAM). The largest game is 8MB in size - which is managed as 512 banks of 16KB each.
There were plenty of machines kitted out with that amount of RAM. S-100 bus and other multi-user systems in the 70's and 80's could handle dozens of simultaneous users. It's cooperative multitasking, not preemptive multitasking.
100MHz. The software you could run! Add a megabyte or so of full speed, pageable RAM expansion. Every computer language right up to C++ (if it works on the 8-bit Arduino it could be shoehorned into a fast 6502 - limited stack? Who cares, just do the big stack in software. Special zero page? Just use it as glorified CPU registers).
What's the point really? But awesome all the same.