I have multiple points to counter and/or mitigate this kind of skepticism:
1 - The odds that multiple EMPs could literally destroy every single electrical grid on planet Earth are very low. Big chunks, sure, but if we assume highest risk of EMP is North Korea vs US - no way they are going to waste missiles on South America (and vice versa) and many other non-relevant countries. Missiles would be targeted on adversaries, not the entire planet.
Therefore, in almost any scenario, some portion of Cardano remains alive and active if we achieve good global penetration.
2 - Cold storage - from the roadmap:
Therefore if many people use this, then at least some Ada is preserved in the event of the almost impossible scenario of no electricity on the planet.
3 - (Future) - some coin was claiming they would have a node running on a satellite in outerspace to further protect the blockchain (don’t recall which one now). Unclear if an EMP (or EMPs) would affect it (based on height of explosion vs orbit and relative distance), but if Ada eventually had a satellite based checkpoint it could be argued this further protects the blockchain.
Anyway my main point is that the ability for EMP to wipe the entire grid of the planet is very, very remote. Advesaries would target each other, not every square inch of the planet for obvious reasons. Even in a nuclear winter scenario, the blockchain could be offlined as that would take many months for things to slowly devolve, and saved for eventual restoration of electricity and restoration of the blockchain from storage.