Dan,
As you well know, the value of the best tournaments often vary by era. Nowadays it is very consistent, with the four majors as the top tournaments but in the early Open Era often prestige was associated top money and certainly the WCT tour and Championship gave the players both. The Australian became almost a minor major at one point. Top players often skipped it to enjoy the holidays. They wouldn't do that now.
True, the value of EVERY tournament varies with the year and the era. Some eras are weak for all tournaments.
But the Grand Slam events have a special meaning.
Historically, the Australian, French, British, and American national titles represented the four premier tennis-playing nations.
Then it evolved that Australia and the USA became the two dominant tennis nations, and the French title became the world clay championship and Wimbledon the unofficial world championship.
There was a natural progression of prestige for these four events, which remains to the present day.
Some of the great pro events, such as Forest Hills Tournament of Champions, Wembley, the Longwood US Pro, or the WCT Dallas final had great prestige, but for a relatively short period.
Because they did not last, they must be considered second-tier compared to the elite four.
The elite four have sometimes been compromised by the pro/amateur split, professional boycotts, lack of interest, but over time they have maintained their special status.
The Grand Slam still exists as defined in 1933, probably the first time that someone set out to win it. The Australian kept its place in 1935, when Perry tried the Grand Slam circuit but was stopped by Crawford.
