You fix at a minimum two sides.

Think about this.

I have a channel with you.

You have 0.1 BTC, I have 0.9 BTC.

I rebalance through you. You now have 0.5 BTC in your side of the channel, and I have 0.5 BTC in my side.

Your side of the channel, and my side of the channel, are both now in balance.

What’s even better is when I do it really efficiently and find another peer who had 0.1 BTC on their side of the channel too. If I use them as the destination channel, I can make their balance 0.5 too, and now all three of us are balanced.

Sure, there might be hops along the way which I cannot see beforehand which may become unbalanced. But they charged me a fee to route through them, just like they would for a normal payment. If that fee isn’t worth their trouble or supposed “risk”, they can raise it or make their channel non-routing.