Does it ruin harem anime to already know in advance which girl wins?
Not really, I had fun when Toradora was airing and I'm finding Golden Time entertaining too.
No, harem anime are already ruined because they are pretty much the shittiest type of anime you can watch.
Why would it?
Pretty much this, OP.
No. If any girl is going to win it's usually obvious from episode 1 who it is anyway.
It actually makes it better because it means a girl actually will win.
Third on this.
I kinda think so. Tenchi Muyo was fun because at one point most of the girls felt like plausible ends.
No argument here.
Toradora was not a harem show.
Even if you think that way, it was still shit.