Interesting, do you think Miyazaki was talking on the practical things you mention, or maybe more about Lupin as a character?
Probably the practical things. Wikipedia says the Animage article was written around 1980, when Miyazaki was done with Lupin permanently. As far as I can guess, it was partly Miyazaki wanting to move onto more ambitious things with Ghibli like Nausicaa, and partly Miyazaki feeling he'd done enough stories with Lupin and couldn't do any more.
The more I think about, the more I am convinced Lupin III works best as a period piece, preferably set between the late 60's to the early 80's.
We had of course our lot of productions putting Lupin in our modern world with good results (I think the recent blue jacket series is the best example) but I always felt like the main Lupin cast always feels slightly anachronistic, either in appearance or behaviour. But that's not always a bad thing. Lupin and Co represent a mindset of adventure, freedom (and moderate anarchy) that is sorely missed in our modern cynical and tense world (at least to me). So to have those characters from an another time helping us to remember something we lost, that's not bad.
I consider Lupin (and his gang and nemesis) to be akin to James Bond. Both started out in the 1960s and are still going strong today, even though their heyday was in their first twenty years and the progress of computer technology has made things difficult for them both.
Like Bond, the characters may be slightly anachronistic (a cop and robber, and a femme fatale, and perhaps most of all a Western gunslinger and a samurai), but I think that like Bond their abilities and personalities stand the test of time and make them not only appealing but a force to be reckoned with.
I would love to see a Lupin adaptation really ingrained in our modern times. It doesn't have to be grim and gritty but a slight re-imagining could be fun. ^^
I think we got that with the Blue Jacket show. It had a British agent working on something that caught Lupin's attention, and Rebecca as a rival to both Lupin and Fujiko.