>>>>"...Turning it to heat is great, but what if the surge has reached a point where it is not normally considered survivable? At that point your customers might already be out of the game, and desperate measures may be justified.
Take an unterminated line (no equipment attached), the surge is going to bounce anyhow, and the customers are going to have to live with this. So (and this is a question) how is this mode of protection any different from the open line?..."<<<<
The idea of incorporating transient protection is to anticipate the types and frequency of transients that might be expected - and to never let them rise to the point where they can do damage. For instance - If I have placed an MDE MAX-260 TVS protector across the power input line of my equipment – any transient on the line will be clamped to below 345V peak within a few 10s of nanoseconds. An attenuated lightning strike would need to have over 288KW of peak energy – or 6500A for over 20uS - to overcome the protection. Granted – if a lightning streamer were to directly strike my input terminals – I would probably be in trouble. But the likelihood of this happening seems somewhat remote. I'm not sure I understand what you mean by an unsurvivable surge - unless you mean one that occurs on an unprotected or underprotected system. I can't think of too many kinds of surges that are worse than direct lightning effects - and we have to protect against those on a regular basis. Some of the testing of protected systems can get downright sporty.
A transient on an unterminated line with "no equipment attached" won't harm anything - there's no customer equipment attached to be harmed. Transients on power lines and open antenna feeds, etc. will still ring down in a normal, exponential fashion - as losses in the insulation system bleed off and dissipate the energy. The feeder end of the power grid will always have some level of transient protection built in. The impedance of the power lines just prevent that protection from being of much help very far from the substation or distribution transformer.