The problem is one of definition, I fear.
A parody is today understood to be something that imitates an original to poke fun at it. However the Kippers, Les Barker and Dave Houlden in his heyday et al are probably better described as masters of pastiche, in that their material is respectful of its influence.
Even in their case, though, a little goes a long way. The thought of an entire evening of parodies would be enought to make me shudder.
George and Giok are perceptitive, I think, in attributing the rash of parodies to the inability of many people to write original material. A parody is easier because the framework is already there; adapting words you already know to fit an equally well-known tune is always going to be easier than doing the full-on songwriting malarky. And, as such, it takes real self-confidence to be able to sing a song that one really has crafted from one's own imagination (and hats off to George for doing that so beautifully).
I similarly know of one songwriter who started by writing parodies - some of them excruciating - but who has now left the nursery slopes behind and has written a number of songs that deserve to make it into the popular folk repertoire. Perhaps it's all part of a process of growing, and in that sense parodies truly are immature!