As Andy noted, the 8088 (and 80188 used in some PC) variant may have been limited to 8-bit bus width. But it had 1MB of address space. And even then, the biggest complaint was the 16-bit segment sizes, i.e. that it took extra effort to span more than 64kB. (The number two complaint was the dedicated registers, compared to the 68xxx family).

I do not think that it is easy to access the memory if you add on a lot of external RAM to a 8051 chip. You ca not beat the simplicity of a nice linear address space. An ARM chip is very inexpensive and several of them have quite large RAM built in. An 8051 has way limited features for addressing more complex data structures. A program that needs a lot of RAM may make use of multi-index arrays, or arrays of structures etc. The DPTR design of the 8051 will be severely stretched - and that is if the memory can be addressed with just the DPTR. Having to start banking the variable accesses will quickly add complexity and have the user starting to fight with the compiler.