A source channel is a channel that has mot liquidity local, because there are not enough routes that find it interesting as an outbound channel with its current outbound fee.
I agree that to be able to have a big discount on the sink's inbound feerate you need big enough fees on the outbound channels routes will use (i already thought about it too, see #537007).
But if you increase the outbound fee of a channel that is a source (that already has not enough routes as outbound channel with it's current outbound fee) to 'increase' the discount, that discount is cancelled by the increase of the outbound fee on the source channel. So routes that come in from the sink, and out by the source channels are still as uneconomical as before.
So the increased discount on the sink only brings something for other outbound channels that actually route with their higher outbound fee.
For the source channel who has had an outbound fee increase, it is actually not encouraging more transaction that would rebalance the sink channel, while reducing even more the few transactions that used the source channel as output, keeping it even more stuck as a source
So for me, the only channels where you could consider increasing the outbound fee to increase the impact of the discount are actually all the channels that are NOT sources

bartoli

Low outbond fees will invite any channel to take that liquidity. So other sources will take it. If you want the high inbound discounts on sinks to have effect you need matching high outbound fees on sources.

SwapMarket

lightning

The Art of Routing Sats

I am not saying this is not the case. I'm saying that in this paragraph, to my understanding, the author should advise the opposite : low outbound fee on the source channel, not a high one . It's the sink that should have a high inbound fee.

For the last part, i might have written too fast, indeed, the inbound discount on the sink channels is good