At first let's consider the typical use case of the Pi (and nod to the fact that its processor is more or less comming from the mobile world). So both the design of the Pi and how most of the users are probably going to use it justify the constraints the current Pi imposes on its GPIO pins.
If a higher current needs to be controlled there are always options to do so, e.g. line drivers or switching transistors, such as the darlington transistor array ULN2803. Those of course come at additional cost and required board footprint.
If optocouplers are desired in the design, it's worth looking into low-current optocoupler as a way to overcome this issue. The use of low-current opto-isolators, operating at say 0.5 mA to 1 mA, would put it well within safe margins even when using all GPIO pins of the Pi.
The Toshiba TLP182 and TLP183 are low input current-type transistor output photocouplers utilizing the SO6 package. By employing Toshiba's original high-output LED, these products guarantee not only a high-current-transfer ratio at the conventional input current of 5 mA, but also at the same current-transfer ratio at the low LED current of 0.5 mA. Especially when utilizing LED current at 100 VAC and other high voltages, these products significantly contribute to a reduction in power consumption by reducing the LED current of the photocoupler. [...]
Vishay - Low Input Current Optocouplers, such as the SFH618A which should be able to be used at 1 mA given its high CTR (Current Transfer Ratio).
PC817 is another generic opto-isolator with acceptable CTR down to 1 mA forward current and might fit the bill