One thing to note about old calculators is that older LEDs had a higher efficiency when pulsed, i.e. having ten times the current at 10% of the time gave more light out than driving the LED constantly. That is no longer true for most newer high-efficiency diodes.

The next thing was that old LEDs could handle very high pulse ratios without a problem. A modern LED has a max peak current way closer to the max average current, which means that having a 7:1 multiplex factor could require that the LED must be run at less than max supported intensity.

For old pocket calculators with LED digits, you normally had to conserve battery power, in which case you could not afford to run the LEDs at max intensity. So even if the microcontroller locked up, the lit segments would normally not get enough current to kill them. When running the LEDs at maximum allowed currents, a software hang without hardware watchdog would be fatal.

Today, LED gives bright light with good efficiency, but 7-segment LEDs are almost only used on mains-powered equipment. Special hardware, such as a timer for taking sequence shots with a camera may still make good use of 7-segment LED digits to be easier to read in the dark. Having a little button to turn on the digits for a couple of seconds would conserve power.