An MMU is pretty much necessary for robust multitasking. Without it, you are at the whim of how well software behaves. Without it, it is more difficult for developers to create well behaved software. That also assumes good intentions from programmers, since an MMU is necessary for memory protection (thus security).

While emulating an FPU results in a huge performance penalty, it is only required in certain domains. In the world of IBM PCs, it was also possible to upgrade your system with an FPU after the fact. I don't recall seeing this option for IBM compatibles. While I have seen socketed MMUs on other systems, I don't know whether they were intended as upgrade options.

By the way, "the i486SX was a microprocessor originally released by Intel in 1991. It was a modified Intel i486DX microprocessor with its floating-point unit (FPU) disabled." (https://en.wikipedia.org/wiki/I486SX)

You could buy a 8087 for your 8086 or 8088, the 486DX just moved it on chip.

That's an advancement but that's a matter of speed an simplicity. An MMU is a huge before and after, it's still the biggest separator of CPUs today. The most important detail to understand a CPU is whether it has an MMU.