Probably poorly worded. I did not mean to imply that you should only create a game engine when your only result would be the game engine, and not a game. You should only create a game engine, when you actually want to create an engine, and not "just" a game. Note sure if that makes it any more clear :)
10+ years ago you did not have a lot of choice when you wanted to create a game, and did not have a lot of money to buy an full fledged game engine. But these days there are really good F(L)OSS and cheap game engines to get you started in game development.
So unless you want to take the hurdles, and experience what it takes to create a game engine, you probably want to get something off the shelf.
But if you want to take the challenge, I recommend to use SDL as much as possible as it takes care of a lot of mundane and finicky low-level platform parts. That way you can focus on the engine technology, and not crack your head on trying to process input from keyboards or controllers (for example).
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