James sat out last night with back spasms. His .265 Win Shares per 48 is his worst since his initial season with Miami, and he has nights where he has "Young LeBron" legs and nights where is does not.
James has logged over 32,000 minutes as a player, plus almost 6,000 more in the playoffs. He is #1 amongst all active players in minutes played per game (39.5), and overall he is #6 on the all-time mpg list, trailing 4 players from the 1960s and Allen Iverson.
With that in mind, we need to look at guys who were really good, who played at least 11 seasons in the league and who were 30 years old and see if ANY of these players maintained a level of play anywhere near even LeBron's career averages of a 28 PER, a WS/48 of .243, and also managed to pay enough minutes going forward to put up at least 20 Win Shares.
The answer is http://bkref.com/tiny/5dHdO. Magic Johnson played about 2 1/2 seasons after age 29 and 11 years in the league, and he played extremely well -- 25 PER, .250 WS/48. But due to illness, he played very little and put up about 35 Win Shares.
Michael played very well after age 29, and self-inflicted some wounds to his career by retiring twice and returning as an aged man. 64 Win Shares. Similarly, David Robinson was still great after 29, but only played enough to roll up 41 Win Shares.
The great old men of basketball are Stockton and Malone, who, despite logging huge minutes and being old, put up close to or over 100 Win Shares after their 11th season. If you sort the list by WS/48, the guy who is the "swing" guy between "still good" and "not that good" is Garnett. Below him on the list is Shaq, Kobe, Dr. J, Moses, Bird, etc. all guys with clear declines after 11 years and age 29. Above Garnett you get guys who continued to be really good with old age like Dirk, Duncan, Barkley, Stockton, Jordan, Robinson.
Kareem is also a special case. 11 years in for Kareem he was 33 years old. He subsequently played 10 years and 26,000 minutes!!!! After 11 years and age 33. His numbers go down, but my lord, when you start counting seasons for guys at age 40, 41, 42, I would hope their production declines at least somewhat, or the league just has become too horrible to stand.
Anyway - the picture for LeBron going forward? It is hard to imagine him playing as long as Stockton or Malone. His game is built far more on mobility than was Malone's and Stockton was a little guy not carrying much weight. I think that Duncan also plays a far different game than LeBron. So I would suggest that you need to look at Garnett, Dirk, Bird and Dr. J. Bird is a nightmare scenario, as he is very low on the list in virtually every category (production and minutes). Dirk has been good and continues to be good, but diminished. Garnett had two excellent years and has been ravaged by injury otherwise. And Dr. J has good solid stats and put up 51 Win Shares, but by 1983 he was a supporting player for Moses Malone and after 1983 Julius's Sixers never did much and basically got out of the way for the Celtics/Lakers rivalry.
So, imagine -- James goes from being a 17 WS a year guy to being a 14 WS guy to being a 10 WS a year guy, plays only 5 more years and slowly gets worse in all phases of the game. That is an AVERAGE scenario for a player with his age and mileage. To win a title, he will basically have to do so this year or have a player on his future team who is first team all-NBA (examples - Pippen for Jordan, Magic for Kareem, Gasol for Kobe).
We probably have already seen LeBron's peak.