If I remember this correctly, research indicates that if the portfolio is being driven by a random walk process, a stop-loss always causes a reduction in returns (negative stopping premium), i.e., stop-losses are useful only if the return generating process can be shown to not be a random walk.

Also, they tend to mess with your mind (loss-aversion psychology, deep feeling of pain at losses) and make you exit and enter the trade multiple times, adding up to transaction costs, too.

All in all, a) Buy and Hold may actually be quite sensible and b) stop-losses are actually much more complicated than you'd think and seem to be a mathematical rabbit hole.

Stop-losses may make sense in, say, an intraday pair-trading strategy where an analysis of past return evolution clearly shows that winners keep winning and losers are hopeless beyond a point.