"The site seems to imply that the clogging is permanent, but I have to wonder. Would a field 'recover' over time if the drainfield is moved elsewhere for 25 years or so? Would a deep equivalent of a rototiller fix the problem?"
Generally yes, clogging is, for all intents and purposes, permanent.
If sludge gets into the leach field, it clogs the leaching material (sand, soil, etc.) around the pipes.
Pretty much there's nothing that will dissolve or digest that sludge -- it's what's left when the anerobic bacteria have had their fill.
The only way to fix the field in a situation like that is dig out the leach field and replace it with new leach media or relocate the system entirely.
"I've seen the 50 year number a lot in regards to the expected service life of a septic system. I'm curious; what happens after 50 years?"
The field explodes and scatters poop over a 4 county area.
Actually, after 50 years the field may be perfectly fine and may continue to operate for years to come.
The estimate is, IIRC, a best guess based on average household usage, moderate, but not high-level, system maintenance, and the recognition that in most systems some sludge is going to get through occasionally.
You can design a system that will pretty much prevent any sludge from reaching the leach field using multiple settler boxes and/or strainers, but maintenance on those systems is even more critical and they're a lot more expensive.
The absolute best thing that anyone with a septic system can do is have it pumped REGULARLY to remove sludge. How often you need to have it pumped depends a lot on the size of your collector tank and the number of people in your household. Two people in a home with a 1,500 gallon collector tank may only need to have the tank pumped only once every 5 to 10 years.
Other things you can do to increase the life span of your septic system:
1. Limit the amount of water you put into it every day. You can overwhelm your system and flush sludge into the leach field if you push a lot of water through your system in a short amount of time. Do full loads of laundry and dishes, take short showers, recycle the rinse water from your washing machine (not the wash water), for watering plants, etc.
2. Don't use the garbage disposal. Food waste is, for some reason, very tough on septic systems. There are disposals that have enzyme cartridges that supposedly counteract that, but they're generally quite expensive and Dad was never convinced that they worked worth a damn. Compost your vegetable waste.
3. Use soaps, detergents, tissue, and other products that are specifically marked septic system safe.
4. NEVER pour chemicals such as paint thinner, lye, drain cleaners, oil, etc., down the drain. They can bring the bacterial processes to a complete stop, and then you are truly screwed.
5. Never flush cooking oil or grease down the system. It can clog intakes or, if it gets into the leach field, can permanently clog soil pores, causing system failure.
6. Keep a good layer of grass over the leach field. This will help prevent water logging from surface water.