Where did this idea that lionfish have no predators come from? I know the concern for the invasive Caribbean lions there were no NATURAL predators, but sharks, eels and groupers are figuring it out. But in their native waters throughout the Indo-Pacific their are numerous lionfish predators.
Triggers are not a lionfish predator. But just because they don't eat lions doesn't mean they won't nip them. Some triggers nip out of curiousity, some out of boredom, and others because they are jerks. Undulated triggers are known to be jerks. And while lions can be quite adept at defending against trigger bites, it is still stressful on the lion to constantly be "on guard."
While personal preferences are important, it is still important to properly care for the animals we house. Adequate surroundings and tankmates are important considerations for the living creatures under our care, more so than our whims.