The tennis tours are not any shorter than they were a few decades ago, and there is now a lot more travel because players didn't go to Asia, or Australia on a regular basis until the 1990's. The women's tour championships were held the first week of November, and the men's were held in the 3rd week of November, and both tours start again in the first week of January.
In 1990, ATP Tour began late December with Hopman Cup, and ended Dec 10 with Grand Slam Cup. (In early 80s, Tour Finals were actually played in January or February next year). There were also more events - ATP had 80 tournaments in 1990, compared to 68 this year. Finally, there were more 5-set matches, as Super9 (or equivalent) events had best of five finals, as did many regular Tour events - and also, in Davis Cup. WTA Tour was practically year-round in the '80s.
Players also generally played more in days of old. For example in 2000 ATP Tour, top10 average was 24.3 tournaments. In 2022, it was 20.3.
Part about travel is probably true - there were Asian events, but less than today, and the tours contained more 'mini-Tours' where players could spend weeks/months at geographically quite compact area (European clay season, American clay season, European indoors season etc).
I also don't know where you're getting the idea that tennis is lacking in young stars. The men's tour has Carlos Alcaraz at world #2 (20 years old), Jannik Sinner at world #4 (22 years old), and Holger Rune at world #8 (20 years old). The women have Iga Swiatek at world #1 (22 years old), and Coco Gauff at world #3 (19 years old).
Last year, three out of four Grand Slams were won by Djokovic and Nadal. In the fourth one, Djokovic didn't play and Rafa was injured. This year, Djokovic won 3 out of 4 Slams. Djokovic also won ATP finals both years. So yeah, when two players eligible for Champions Tour win majority of Majors, I'd say Tour is lacking new stars. Alcaraz, I agree, is a proper star, but before he came along, ATP was looking about as credible as WWE or boxing.
There are several factors that have led to longer careers:
- more money available to more players allowing players to have access to physiotherapy, and coaching
- age restrictions that limit tournaments for players 16 & younger
- the game is more physical requiring more mature athletic bodies that young teenagers don't tend to have
- depth of competition has increased as the number of countries that have players at the top of the game has increased
No disagreement (except the last point, I believe depth of the Tours is susbtantially less now than it was 20 years ago, as young athletes tend to go to team sports).
One factor is that balls and courts are slower nowadays: many people (including myself) thought more grinding playstyle would lead to more injuries, but turned out it was the opposite. It was explosive movements required on fast courts which caused more injuries.
Nowadays, tennis has also tried to root out "tyrant parent coach" phenomenon, which produced many famous teenage wonders. I won't name examples.
Lot more attention is put to physiotherapy and muscle maintenance, as you say. There are also persisent rumours that hormone use to heal small injuries is very prevalent nowadays, but that is just a rumour.
Is any of that applicable to figure skating? I don't know...