i'm not sure what you mean by this, the type of the locktime (being height or timestamp) doesn't change anything about safety of it. they are both enforced by the protocol and work pretty much the same way.
in fact for longer periods such as 20 years using the timestamp may be better since we can have more blocks or less blocks per day that can accumulate to a big difference in 20 years while timestamp can't change and has a very small wiggle room.
Dam now you've got me thinking the blocktime interval might be reduced at some point too...
And yeah I guess the largest miners would lose access to the mempool before they lose access to timeservers and become out of time...
Only if that exchange still exists after I pass though! Most exchanges seem to refresh wallets often enough, it'd be another hell to fix it with customer support. As long as they're somehow with the keys I suppose they could always seek help from someone. Thinking of making all KINDs of instructions or really just finding a way to put it in a will and update it as much as possible.
Oh yeah... I was going off coinbase still accepting my same address 6 years later but you probably could find a solicitors/attorneys that will allow you to at least leave them a transaction to broadcast at some point, or you could hide hardware somewhere that's listed on a will.
My ideas are normally reliant on someone giving a friend access as my family would have no idea what they're doing and would probably end up losing stuff...