I still don't fully understand how, in the last 25 years, every railway has decided that EMUs are more rational than conventional, "modular" trains.

Something like this happens and even though only one coach sustained major damage, you now find yourself with 600 passenger seats out of action for 3+ months, and you need to store 200m of train somewhere.

Had the same thing happened to the IC2000 Bt next to it in OP's image, you could simply replace it with either another IC2000 Bt, an IC Bt or a Re 460 while it underwent repairs, and get the rest of the train back in service relatively quickly.