Dak couldn't be less like RGIII. He's very smart, he works hard, he's humble, and he can actually throw the ball. RG-ME was never a great passer, but his absurd running ability covered that up. Once he got hurt, he was exposed. On top of that, he refused to let people help him become a better player because he believed (and still does based on quotes I heard recently) that he is the best quarterback the planet earth has ever seen.
I don't know as much about Kap, but he clearly lost a tough of muscle when he became a vegan, and his game seems to have dropped significantly since then. I also don't believe he is nearly as smart as Dak, and I would point to the contract he worked out for himself as evidence. Add in the circus caused by him deciding not to stand for the national anthem, and he has become more trouble than he's worth.
There is no way to know how Dak's career will play out, but based on everything I've seen him do and heard him say up to this point, I would bet on many years of success.