There is demand, but it's niche and depends heavily on the model. An iPhone 6s or Galaxy S5 - those aren't rarities anymore, more like "familiar classics," and prices for them are stable but not growing. But genuinely old models - the first iPhones, early Galaxies, rare releases - there's actually demand for those: collectors, people chasing nostalgia, tech history enthusiasts. They do buy, but for specific pieces, not just any random old phone.
Prices on mainstream models like yours have basically plateaued. An iPhone 6s or S5 sells for peanuts compared to new devices, because they're just not fast enough for modern tasks anymore, batteries have degraded, software updates stopped. Prices only really go up for truly rare items in pristine condition - gold versions, limited editions, variants that were rarely released in your region.
If you want to sell, focus on condition (scratches seriously tank the price), completeness (the box and papers boost value), and model specifics (a rare color or storage capacity). Marketplaces will help you gauge actual demand in your area - check what's selling and for how much. The most active buyers right now are people looking for "a working phone without internet" or trying to relive nostalgia.