You are right, you could "downgrade" to regular seating if there's an empty row. The only things is, what happens if the flight's full or almost full? From personal experience, there have very few empty seats on my recent Frontier flights.

I guess I just have a hard time reconciling the idea of being one of the very few Elites on Frontier and ending up in a middle seat for 5 hours.

Should you choose this strategy, good luck!