Stage 1. There was virtually no change prior to 1975.
The women already at the very top had, by definition, enough independent wealth security to stay at the top and the sea of women below were kroll moving around the lower levels unnoticed until they died off or married someone who could and would provide them the security to pursue a shot at the surface.
Stage 2 By mid 70's, money from Virginia Slims, began flowing in to support and the impact of a WTA brought more stability throughout the top ten to 15 or so who could reach the quarters or semis often enough to stay on the tour throughout most of the year and provide some depth, while fame from television coverage offered the first real sponsorship deals to the stars and more security to the venues being promoted on television. Meanwhile careers were longer at the top secondary to the immeasurable impact of The Pill and safe abortion access. Players were able to plan pregnancy and maternity
Stage 3 late 1970's was when the first generation of young girl stars like Austin or Jaeger who were be seen as money producers for their respective families or as national assets across the Iron curton like Mandlikova, Sukova etc, as opposed to drains. Coaching camps, national development programs now had the dollars behind them to groom talent consistently at all levels. That brought significant improvement to quality and visibility of the tier 2 women competing and there was enough start 'star power' and money flowing for a dramatic explosion of events on the calender. A generation of mothers as well as fathers, could also provide income to the household, as the women's movement offered more chances for steady income outside of a very narrow jobs.
Stage 4 now the impact of title VIIII was felt in the states with more support for tennis programs in schools and college programs but the number of events that television could support saw its limits . The purses in the top three rounds soared as tournaments began a desperate attempt to attract stars to maintain viewership levels in the face of saturation . Top women could afford nutritionists, conditioning regimes, and access the very best sports medicine money could buy. The power of the WTA was complete and total . the Grand Slams became required attendance, and that same television coverage put more pressure in improve officiating.