That just means that you need twice the programing effort. Part one is a game play engine, part two is a game content creator engine. Then of course your need to attract people who want to create a game but are not programmers. Of course to create a game of that type needs some programming skills, but the level is - or should be - a lot less than writing the engine.

A good game engine with good tools can save users who want to write that type of game a lot of effort. You shouldn't have to write collision detection algorithms to give the user a sword/gun/magic spell.