GPUs can be considered as active capital. You always can sell every part of your rig or use it as powerful PC. Outdated Antminer is just a piece of metal and chips.
Yes, but an Antminer S9 is vastly more profitable than any reasonable GPU rig. You're likely to earn about the same from an Antminer S9 right now as you are with a GPU rig with 6x 1080 Ti's. That GPU rig would be a lot more expensive though.
So yes, while GPU's have better second hand value, an S9 is way more profitable.
Quick rough estimate from Swedish current prices of GTX 1080 Ti vs Antminer S9:
6x GTX 1080 Ti (Swedish market, cheaper in other countries with less taxes etc): $5226
3x Antminer S9 from Bitmain: $4245
6x GTX 1080 Ti mining BTX per day: $28 give or take some depending on card, miner and OC.
3x Antminer S9 mining BCH: $83
Since it's not possible to calculate proper price in the future, let's just play with the thought that the difficulty and price will stay the same. So in 6 months, those six GPU's will have earned you $5190 while three Antminers will have earned you $14982. That's quite a big difference. Granted, you'll earn less depending on your electricity cost.
Cost of GPU's are just quick prices in Sweden. They'll be cheaper in other countries. Then again, I didn't include any other components needed, like CPU, RAM, PSU, etc. Same goes for Antminer, the PSU is not included. But who knows, anything can happen in the future.