They're plenty fast for DMX (only 250kbaud), that's not the issue. Just not an appealing solution these days, generally. One 14-pin digital isolator can replace four optocouplers and a handful of resistors, less board space and maybe cheaper.
There are some all-in-one solutions for power+data isolation, including parts that have an entire isolated DC-DC converter, signal isolation, and RS485 transceiver in one PCB-mount module. There are also some general-purpose isolator ICs that have a built-in DC-DC controller and just need an external transformer. I tend not to use those personally just because they tend to be single-source parts, so you can find yourself in a bind if they go out of stock when you need them.
Really the only advantage to optocouplers in my book is that they're easy to socket, so they can be replaced in the field if damaged, but if your isolated DMX port takes a big enough hit to fry the optocouplers how much else is gonna be toast? You can socket the transceiver IC as well, but I'd rather save the board space and parts cost and put that towards front-end protection. Transceiver ICs tend to be fairly robust, so you don't have to go crazy to protect them against the most likely fault conditions. Anything that goes beyond that, and, well, that's one of the reasons our DMX interface is a separate small PCB
Re: the other stuff, I tend to just spew information at the slightest provocation, but I'm happy to clarify anything if there's something I didn't communicate very well.