It's very rare someone so succictly directs a post at someone else that's best directed at themselves.
Engineers is "optional" in the sense that not paying your taxes is "optional". Yes, you can, technically, blow off April 15th like it's nothing. Nevermind the fact that angry people with guns will eventually show up at your front door, the fact that you can do something is less important than the consequences for doing so. Or something.
Unless you intend to hang out in Solo or PGs forever (or move to a country without an extradition treaty, going back to the previous example), they are effectively not optional. Reason being that every nolife with an axe to grind has engineered the hell out of their gear, and you will be ganked, and there is very little you will be able to do about it even with A ranked kit.
"Optional" with serious consequences attached is not truly optional. And I think you knew that, and would just rather snark at other CMDRs.
Of course Engineers are optional to use or not use. It is also optional to play in Open/PG or Solo. You can still play in Open without any engineer mods, but, it might not be recommended to do so. But since the game have three game modes, Engineers are, indeed, optional.