but how "baked in" to the protocol is locktime? ie it will forever be compatible with whatever official client is in use in 15 or 20 years time? a tx created with locktime today will always work in the future?
In 20 years a lot can happen, so this could indeed be a risk. Or, maybe even more likely: a potential protocol change could invalidate the transaction too, for instance if quantum computing becomes a threat to the current encryption.Unfortunately, we can't know for sure, so don't put your life savings at risk
And there's this:
Risks
You may miss out on possible Forkcoins that use proper replay protection.
You may miss out on possible Forkcoins that use proper replay protection.