Yeah, if you think you can write your own alternative to Unreal Engine for less than (revenue-$3000)*0.05 per quarter, then by all means, you should do that.
iDTech is no longer licensed for third-party use; only Zenimax properties can use it. CryEngine is available -- they have a $10 a month subscription plan that is royaly-free, but you don't get source code access like you do with Unreal Engine here. (They offer seperate terms for source access.[1]) Unity exists, but I don't think it's in quite the same tier as Unreal (although for some projects that may not matter).
I can't build a decent car for less than the cost of a low end BMW. But, there are plenty of other companies than can.
So, the real question is what are the other options out there and I suspect there are few engines that can come close.