Whether its a power boiler or a recovery boiler there are a huge number of sensors and actuators needed to monitor and adjust. Just the pressure systems at the various points inside the boiler tubes. Required miles and miles of copper cable. There is also a huge number sensors and actuators in the fire box from the fuel feed to the exhaust and anti pollution systems.

Then you move on to the turbine section and you have more wiring for that.

I once saw the aftermath of a single oil sensor failing and not going to zero. It hung and didn't alarm. The the babbit bearings on the turbine side seized. The whole place nearly melted down until the engineer on site managed to get the boiler shutdown with very minimal damage to the boiler. They fired him the next day for the maintenance miss and they had to rebuild the whole turbine because the blades got wet.

Power boilers if they lose water they will melt from the hanger to bottom. I've seen the aftermath of that as well. So the old legacy stuff is here to stay until it stops working.

Nukes were that times ten. Multiple sensors for any given system plus stand by emergency systems all leading to control room. Inside the containment vessel there are cable trays everywhere on every level. The support equipment outside the containment vessel were no different. It is unlikely most of that will can ever be replaced. Many of these systems take a decade or more to slights retrofit because with a nuke you can't really make a mistake.

I sat in on multiple meetings because a pump housing mounting bolts were supposed to be left handed threads but were right handed instead. It took them over a week to decide if it was okay that way. Engineering office guys were trying really hard to not to have to submit all the paperwork to get the prints changed after nearly twenty years in service. It would have been very costly in time and materials to remove the studs in the floor and replace and repour the floor section so they kept the right handed threads. This should give you some idea how hesitant they are to change anything in a nuke. I can only hope in the nearly thirty years since I did those kinds of shutdowns they have replaced some of it.