The electronic dimmers were packaged in racks and racks of drawers, and it was a simple matter to repair a vital channel by moving a drawer. The 1966 vintage Strand Electric console had sufficient internal parallelism that we lost channels and pots, but never the whole thing. The lighting system failed occasionally, but never took more than an hour or so to restore to operation, since we stocked a full board replacement inventory. In my five years there it never failed on air. I see current lighting control systems at the television trade shows.
Smaller systems build the dimmers right in.
The control desk contains a microprocessor that operates each channel, and reads levels from a floppy disc. The key to making these systems redundant is buying two control desks. Individual dimmer channels can fail, but that won't shut down a show. The amount of rehearsal needed to choreograph the operation of a manual lighting console is significant. A failure of the modern control desks means the system is down, since there aren't manual controls any more.