A lot of the flight subsystems are rerely turned off, and are running all the time between services, and often also during service pperiods, as the plane will be powered from an external power unit then. Reboot of some systems can be fast, because it is a warm reboot, but powering down, and doing a full restart can take hours, as you have to bring things back up in sequence, so you do things like first connect the main battery, then turn on the GPS and wait for it to acquire lock, so your main data bus systems will have a stable clock and accurate time data. Then you can start powering up the systems in sequence, such that each one communicates with the units it depends on to get valid inputs. finally you can power up the FMS and displays, which is the top layer the pilots see.
That at least reboots fast, but only if the underlying systems are running, without having exceeded the abilities of the redundant subsystems. Things will fail all the time, and there are a whole lot of fault trees that you follow, to see if you can continue to destination without degradation, continue with degradation, divert to alternate airport soon, land as soon as possible, land immediately or " kiss your backside goodbye, because that is it folks", depending on the fault and just how many systems it affects. Then you get mechanical failures, with the same fault trees, but at least you can hope the electronics is going to give you a true indication of the system status.
51 days of uptime then it breaks is a symptom of poor testing, just like the engine management turning off the alternators after 250 days of uptime. Both of which are common to have, as those systems run all the time, even with the plane on the ground in a standby mode. Likely to have been introduced as a consequence of another software bug being fixed some time ago, or to be a result of errors in interpretation of inputs, or validation of parameters, or just insufficient testing of all possible program paths.
You might turn off the lighting on the ground, and also turn off the APU, and have a cockpit that is totally black, but there are still computer systems running to enable you to turn on a switch, and have the lights come on, and it to warm start in the morning, but the uptime on the computers will be that since the last time the battery was disconnected along with the external power connections.
Only good thing is all the grounded 737MAX aircraft will have been parked with the main battery breakers turned off in the avionics bays, so as not to overdischarge the batteries, or they were removed so they could be put on maintenance charge schedule. They will be fine for the first 51 days after they fly again, at least for this bug, and perhaps even MCAS issues will be properly fixed as well.
I will add that military aircraft rarely are powered for long times, as they have in general a much more frequent service schedule, due to parts being run much closer to the limit as compared to commercial aircraft, where they want parts to do long periods between services, and even longer between overhauls. Thus your jet engine might run 90 days between service intervals, or 500 running hours, while your military version of the same engine will be monthly, or 200 hours, due to the higher demands placed on it. Even in hot standby the same aircraft will rarely be run for more than a week continuously, before another is swapped in to replace it, while the first goes for service. In a situation of active military use though they might just be run till they break, but that is where you find that they are also going to be spares part sources quickly, to keep others running.
Longest I saw was 2 weeks of continuous operation, in use 8 hours a day, with hot swapping pilots, because they were down to only having one aircraft available, the others being not available for various reasons, and the pilots needed the hours to keep flying. It did not break, though the engine came in right afterwards, well past the service hours count. I had no faults off it either, the avoinics i was responsible for worked correctly.