The problem is that what is holding Scrypt back is not what was holding SHA-256 back. You can throw more and more processors at SHA-256 and come out with more and more hashrate. This is simply not true for Scrypt. Scrypt requires more processors combined with a higher and higher memory bandwidth. Nvidia and AMD place a lot of money into memory R&D to that end, more than any person interested in a Scrypt ASIC can currently field.
Basically, a Scrypt ASIC will not hash faster than a GPU, because you're not going to put >GDRR5 memory on an ASIC. However, a Scrypt ASIC could work as a standalone unit, which would be beneficial for someone who wants a lot of them. Other than that, there is no real benefit to designing a Scrypt ASIC for anyone interested in cryptocurrency.
You are forgetting about the power costs.
Since ASIC's are so much more efficient than using a GPU, they use a lot less energy.
Say an ASIC and a 7970 both hashes at 700KH/s, but the ASIC uses only 80 watts while your 7970 uses 300 watts.
That will save me around $25 a month per ASIC (it's $0.16/kWh where I live)
If I get 12 ASIC's, I'll hash at around 8.4MH/s and save $300 a month.
That's quite a lot when added over the lifespan of my mining rig.
