Sure, but there are also different problems. For example, a stepper motor hooked up to a bit-bang SPI bus that needs to be advanced 2000 times per second. You don't want too much jitter, so doing it in the timer ISR itself becomes a viable option.
It would be nice to have the option so you could choose to run the ISR in assembly, or a higher level language, depending on the particular circumstances. In my experience, I've written 90% of my ISR code in a higher level language.
If it only takes a half-dozen instruction to do a context switch, I'm sure it doesn't have to take more to set up an environment to run Forth in an interrupt context.