well the point is not to let people disable defender and then never be protected because they never enabled it back. it makes sense...
again if you don't want that kind of automatic protection you have two options.
1. upgrade to pro, and then have more control.
2. I am sure you didn't like my first option but also you can get a free cheap antivirus and disable that one, and never let it on, you know maybe something like Qihoo 360 Total Security Essential. it will disable defender as antivirus, and then you can close it if you don't need it and want it or use only the sandbox or nothing, but it will serve as a way to disable Defender and don't let it run again like it does by default.
I really don't even understand why you exactly want to disable it... of course it's on because you can use it without having real-time on and because it will reactivate after a while.
but it's only 25mb.... it does nothing but stay there sleeping and waiting. and I am sorry but 25mb is nothing for today's devices memory, 2gb or more.