But the resistor in front of an opto isolator is just as likely to short circuit as a resitor in front of a MOSFET, so you are not anymore protected from a short circuited resistor if you use an opto isolator as if you use a MOSFET.
I used to repair electronic devices for a living for several years and short circuited resitors is a not issue at all really - in reality it hardly ever happens, I can't rember seeing short circuited resistors. Wirewound resistors are maybe more prone to this, but they are not used here, but only for some power applications.
An opto-isoltated circuit does however offer more protection, it can protect from reverse voltage and large over voltages. But you don't need an opto isolator to simply protect from a short circuited load transistor. The opto-isolator could short circuit the inputs pins too and draw a lot of current. But a resitors in fron of the opto-isolator or load transistor could prevent overloading the output port in case of a short circuit.
If you want opto-isolation and MOSFET you can use a Photo Voltaic Relay as suggested in my earlier post. This is a good and small solution and I think the price is about the same as a regular realy, while it's much smaller and doesn't need a transistor to drive it if your output can supply enough current for the LED in the Photo Voltaic Relay, and you don't get the clicking noise of regular relays.