1) The source engine is already showing its age. (see: constant patches for any Valve game released in the last 2 years)
2) Valve, unfortunately, seems to be going the Infinity Ward route and will try to release poor re-hashes of its original franchises until it looses most of its customers.
3) If more people complained (see: L4D2 boycott) about shitty coding, perhaps they would bother to make a new engine. And by "new engine", I don't mean "Source patch 1000,1000,1000.1".