Yes, I think that this is - for the most part - true. The picture is not always uniformly black & white however. Some carriers now allow you to extend your miles validity in return for a fee but whether or not this is a financially worthwhile ‘solution’ is of course a matter of personal judgement.

And in some cases, you can avoid losing the miles by changing your booked flight to another date, rather than cancelling outright. It seems to me that for a long-haul flight in a premium cabin (ie one that involves a large number of miles) it would make sense to do this - even if it means picking a new flight on a ‘random’ future date, which you may or may not be able to actually use. If it then turns out that you can use the re-arranged flight then fine ; but if you cannot, then yes you will lose the miles BUT at least end up no worse off than you would have been in the original cancellation scenario …….. If you get my drift !