The resale market for the high end GPUs for gamers remains strong. just take a peak on ebay. When these GPUs come out and miners buy them in bulk its the gamers missing out (just go to a PC gaming forum, they do NOT like miners. lol). So after 12-18 months these gamers can get these video cards at 1/4 to 1/2 the price they usually jump on that. or at least to this point. Either way, regardless of what you can get GPUs have a function outside mining, ASICS dont… take that for what its worth.
As far as the difficulty on ASIC mined coins growing faster then GPU mined coins… this is kind of a generalization as each coin has its own difficulty rules. but ill give a general example. keep in mind, the awnser to this is much more involved, and i can explain in more depth if you like, but a psuedo overview is this.
mining is just solving blocks of math problems/algorithms. each math problem/algorithm is more difficult then the next.
so once you solve the first block, the next block is more complex and takes longer to solve (mine). the one after that even harder.
now… the quicker you solve the blocks, the faster you get to more difficult blocks. and with ASICs have huge hash power and solving blocks at a faster rate then GPU miners, your getting to those next more difficult blocks at a faster rate.
Asics coming on the scene and mining tons of coins and blowing through blocks quickly is a big reason why alot of PoW coins have tried to become asic resistant, or even move to PoS instead,
Ethereum for example is a coin that is ASIC resistant. the reason for this is that ethereums algorithims are extremely memory intensive, and you need to load a header for each algorithm into memory (called a DAG). as difficulty increases, this DAG gets bigger. so while ASICs have a ton more processing power then GPUs, they are bottlenecked by memory, which makes then no more efficient at mining ethereum then GPUs. it is not cost effective for miners to add tons of memory ot their product, which is why at least for now you havent seen ASIC mining for ethereum.
there are challenges for difficulty with ethereum/GPU mined coins as well. for instance once the DAG reaches epoch 199 or 200 it will no longer fit in memory for cards with 4 GB or less memory. which will no longer be capable of mining ethereum