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As for Bill being no spring chicken, since when has some advanced age been an issue for Kung Fu experts who have a few wrinkles and grey or white hair. Pai Mei didn't look like a spring chicken either. Some of the most famous martial arts characters have been older than Bill.
We all grew up with Mr Myiagi in
The Karate Kid
I disagree with you on a number of levels there.
Maybe I'm misunderstanding you here, and while I can accept fight scenes between older opponents vs younger opponents, one involving David Carradine would not have be all that spectacular.
For real-life martial arts experts age is still an issue, because they're only human like the rest of us. While experts who train sensibly and consistently can be very agile, quick and even powerful into their 70's (trust me, I've met people like that) they still feel the effects of age, and show it too !
Interestingly in 2012, at the age of 58 Jackie Chan announced he was cutting back on his stunts and trying to look after his body more ( look at his stunts in the awful 2010 Karate Kid, a lot less impressive even than his stuff in the last Rush Hour movie, in 2007).
As for Pai Mei, Gordon Liu, the actor playing him was a life-long wushu expert, who had been making Kung fu films since 1974 (see my comment above) with the benefit of wires and who was around 50 at the time, Carradine was nearly 70. Also, Liu has really toned down his stunts as he's aged.
As for Pat Morita, Mr Miyagi, well he was also around 50 at the time Karate Kid came out (again, a lot younger than Carradine). As someone who knows a little about Karate and Martial arts - having studied for nearly 30 years, I can tell you honestly that the Karate demonstrated by both Pat Morita and Ralph Macchio was pretty average (and that's being nice). The Karate Kid is one of my all time favourite films, and I still love it today, but I can tell you with absolute certainty that in a real fist fight William Zabka (Johnny, whose Karate wasn't all that bad) would have destroyed Macchio, crane or no crane.
The Pat Morita v John Kreese and Terry Silver fight scene in Karate Kid 3 was rubbish (not bad by 80's mainstream standards, but by today if you watch it, it's not good). Even Miyagi vs the Cobras in Karate Kid is pretty average. Back in the 80's martial arts choreography in mainstream cinema wasn't what it is today - hell, watch any Van Damme movie from that time, and keep track of all the slo-mo, and he was a real martial artist (again, not like Carradine). Even the Daniel San v Cho-Zen final fight in Karate Kid 2 is not great.
What IS great about the Karate kid movies (and the first 2 are great) are the characters the story the pacing and everything that builds up to those fights - so we can easily forgive mediocre choreography, so long as the good guy wins.
And we're not talking about Gordon Liu, Jackie Chan or even Pat Morita here, we're talking about David Carradine, whose kung fu is pretty crap at the best of times, who was an alcoholic and regular drug user and a pretty average actor.
If you watch his TV series from the 80's "Kung fu the legend continues" lots of his kung fu scenes were slo-mo, to hide how **** his technique was.
Chuck Norris (who fought Carradine in Lone Wolf McQuade) famously said that Carradine was " about as good a martial artist, as I am an actor."
What does that tell you ?
I've seen the Deleted scene with Michael Jai White (who is the real deal) and if Carradine looks decent there, it's because White is making him look good. If you watch that scene carefully, Carradine doesn't do anything particularly impressive (he doesn't throw a single kick, for a very good reason, his hips would probably give out).
So, to sum up, while in principle I have no problem with an older person fighting a younger person in a film (and I can accept that non-martial artist movie stars can actually put on pretty impressive displays e.g. Keanu Reeves, Laurence Fishburne and Uma Thurman) I still seriously doubt David Carradine would have been impressive in a protracted final confrontation in Kill Bill Vol 2.
But hey, that's just my point of view.