You'll also see that all car manufacturers use extensive plastic cooling parts now. It's certainly an issue in the long term but 20k miles and just a few years would be very unlikely

 

I think you are suffering from the internet effect of too much information. People come to these forums looking for solutions to problems, but you're getting a false impression of the true statistics. Nobody ever makes a post just to say "Wow, my car's cooling system has lasted 200K miles and I've never even opened the hood let alone changed the fluid." There's probably a whole lot more Jaguar owners like that out there than those of us on this forum who tend to obsess over our cars. So when researching a used car on the internet, just think of these posts as things to be aware of and plan preventative maintenance around rather than things to worry about. If you know its a possibility, then you can either plan to have all the PA66 nylon cooling system components replaced at say 60K or 70K miles, OR you can just keep your eye on the coolant level and fix it at the first sign of trouble, maybe buy an extended warranty if you have trouble budgeting for a large repair bill. Either way, knowing about it ahead of time will mean that you'll be much less upset about it when it comes time to spend the money on it.

 

My grey jag that I bought used had on the carfax a coolant pipe replacement after just 2 years and 20K miles.

 

Climate has a lot to do with it. My XF came from Texas so even after 4 years it was clear that plastic pipes like the return hose from the Y pipe to the expansion tank had probably deteriorated somewhat more than a car being driven in (eg) London.

I would also agree however with the sentiment that despite the rather alarmist title of this thread, and the fact that this issue is brought up a lot here, it's not a reason to avoid buying a car, just to be aware that as cars and engine designs progress over time, there are more things you need to consider as potential maintenance items in the future as opposed to something that might previously have lasted a lot longer, along with other "new" service items that are also shared with other manufacturers such as intake valve cleaning which prior to DI might never have been needed in the lifetime of an engine.

Certainly not a reason to be worried about driving the car though. Not much more of a worry than failing coolant pumps, I'd have thought.