The strategies might need to be different depending on the program(s), even the territories. The OP is in a part of the world where un-extendable mileage expirations are common. Most of the replies, however, are from the US, where (except for some of the LCCs) most airlines allow to you to extend expiration "forever" with any "activity". So the concern is devaulation (or the carrier disappearing!) after the period of time that the miles would have expired in the OP's program.

I'm not concerned that AA miles are going to disappear, or tremendously lose value. I am concerned that they'll stop treating all AA miles earned from all sources toward Lifetime Platinum before I get to 2MM. (They're the only airline I know of that counts all non-flying miles toward lifetime status.) Meanwhile I want to maintain this Platinum status until I earn that, and need to earn some good chunk of it still by flying. So with that goal in mind, I "can't afford" to burn anywhere near most of my miles right now, because that would mean not getting enough paid flights in (virtually all my flights are leisure, yet I have limited vacation time each year, and no interest in "pure" MRs).

If I weren't collecting (primarily) with this one airline and trying to get to that one lifetime status point, my strategy might be totally different. And if I were (by nature of where I lived and/or by nature of where work travel took me) only able to collect with an airline with "hard" expiration policies, my strategy would be yet different again.

So I'm not sure how well it can generalized across all airlines everywhere in the world...