There are many metrics for determining the success of player development programs ranging from the number of players participating in Grand Slams (quantity) to the performance of players in the Grand Slams (quality).

Participation (Quantity)
Based on participation criteria, entry into the main singles draws, the 2019 Slams were a major success for the Americans, as they filled 217 slots, 14.1% of the total, in the men's, women's, and juniors' events. Seventy-three slots were filled by American women and almost one-third of the junior girls at the U.S. Open were American.

Performance (Quality)

If success is measured by performance, such as reaching the quarterfinals, the 2019 Slams were a colossal failure for American tennis.
There are many talented American players, but in 2019 none of the American men, women, or junior players won a Grand Slam singles event. Twenty-three of the 217 slots filled by American, 10.6%, resulted in players reaching the quarterfinals or better.
The American men were not a force in the 2019 Slams!
Stephens (age 26) and Keys (age 24) continued to be underachievers in the Slams as they only reached the quarters at the French Open. Riske (age 29) and Collins (age 25) surprisingly reached the quarterfinals at Wimbledon and the semis at the Australian, respectively. Will these four women improve their 2019 performance in the 2020 Slams? What other American women are poised to reach at least the quarters of the Slams in 2020?

Anisimova (age 18) had the best results of the younger women players, reaching the semis of the French Open. Kenin (age 20), Gauff (age 15) and McNally (age 17) created excitement and hope for the future, although they did not reach the quarters in the women's Slams. Will these players continue to develop their skills and be counted on for stronger performances in the 2020 Slams?
In the Slams juniors events Kingsley, Nava, and Damm reached the quarterfinals in the boys' events and Navarro had two strong performances in girls' tournaments. Are our player development efforts only able to produce a handful of players who might be able to compete on the pro tour and reach the quarters of the Slams?

Participation or Performance?

The Grand Slams are the Super Bowl of tennis. The cold, hard facts are that the Slams are professional sports and a form of entertainment designed to generate revenue and determine winners in the men's, women's, and junior singles events.
There is a strong rationale for having a player development program based on the quantity of ranked players or number of participants in the Slams.
Right or wrong, the primary measure of success for professional athletes is performance. Specifically, many tennis fans can tell you that Serena Williams has won 23 Grand Slams in her career, but it is irrelevant to them that 22 talented American women participated in the 2019 U.S. Open singles event. Participation is important in developing a base for player development, but the ROI is measured by championships and strong performances in the Grand Slams.