The fire risk is real
Dryer lint is one of the best fire starters there is — campers carry it on purpose. Pack it into a duct with a heat source at one end and nowhere for that heat to go, and the outcome writes itself. Failure to clean is the leading cause of home dryer fires.
U.S. fire departments respond to an estimated 13,000+ home fires involving clothes dryers every year, and NFPA traces roughly one in three of them directly to lint and vents that were never cleaned.
Most of these fires start in a vent nobody had looked at in years, in a house where everything else was maintained.