I've kept bristle nose with swords for quite a while--I would suspect that either A) something else killed the sword (low light, lack of nutrients) and the pleco just cleaned up the dead leaves, or B) the pleco isn't getting enough to eat. They really don't like healthy plant matter much--I've never seen mine touch a healthy plant.
However--one tough leafed plant you might want to try would be the large caffeola crypts. They look similar to swords, but grow from a corm instead of a rooted mass, so should be 'planted' in loose substrate with the corm exposed. Anubias would be another good option, also a lower light plant that is easy to grow.