How fast does new technology "trickle down" to lower models?
They say that this year's Tiagra is generally last year's 105, which is true to an extent. You'll see things like 10 speed cassettes move down the product line at a rate of one series per year or so.

It's not entirely true though. Features creep down, but quality stays about the same. The different series will maintain their relative build qualities year after year. I.e. a higher priced series will have better materials used throughout.

Having said that, I don't believe that build quality is really a deciding factor in most groups. I've put 20000km into a Sora group that is still running just fine. You just need to keep up the proper maintenance.