There are no guarantees- injury, bad supporting players, bad games. He is already receiving headman hype and considered a possible first round QB. Take it and run even in a crowded class.
If it doesn’t pan out and you fizzle at the end on the season and your draft stock falls, go ahead and take another year. At that point risk reward is about even. There will be competition in the position every year, new younger bigger faster stars. Holding back just changes names. If you’re 1st round and finish well, take it.