Words changing spelling over time is entirely different to changing the fundamental meaning of a phrase lol As David Mitchell said, forts are not giant inflatables that you need to hold down (with the possible exception of actual inflatable (bouncy) castles hehe).
So you've never had anyone tell you to hold things down while they run an errand or something? That is, to ensure chaos doesn't ensue upon their departure?
Words change meaning, which can change a phrase. My comparison to spelling had to do with what defines something as right or wrong. If more people do something one way than the other, does it make the minority wrong? Or does the majority remain wrong? Personally, I say you can't judge either as right or wrong, but rather it depends on where you live in some cases, just as you wouldn't come to the south to tell someone "ain't ain't a word ya idjit!"
And I'll agree, not everything makes sense. There are plenty of holes in the language we use. 'Irregardless' easily comes to mind.