I suspect that McCartney knows exactly what he's doing at every step in the music business. Each decade seems to see him collaborating with someone that's still fresh in the market. McCartney just grows his fan base by the generations, rather than milk those that got him started and established. I also think McCartney knows what the people are like that he works with. He's been in the business long enough....hey, he had to endure Yoko and John.