Everything comes with it's own set of trade-offs, on-chain BTC is best security in the world but with great cost and low scalability, lightning is great security, infinite scalability, but still big cost (funding and liquidity management), ark is giving up some security for bringing the cost even lower for allowing even smaller tx's bypassing liquidity management problems. The way I see it is on-chain BTC is for very large tx's, Lightning is for medium tx's, Ark is for small tx's. But you can use or not use anything you want, that's the beauty of open and permissionless systems like bitcoin.