It's unlikely for a small indie to roll a mobile game engine But if you are talking about companies with millions of dollars of income from gacha games like Hearthstone, most of them have already attempted to roll their own or even used in production already. (NeoX from NetEase, Cyllista from Cygames and Supercell’s unnamed engine…) For mobile game engines, they are not striving for crazy visual fidelity since mobile hardware is really limiting. Those who stick with Unity now because it’s cheaper, but if Unity tries to charge way more, they will definitely spend that markup to push their own tech onto production instead of surrendering money to Unity.

Unreal has technology moats like Lumen and Nanite to justify the royalty while Unity doesn't.