Clay Court Pickleball
This sport appears to be the one that seniors love because of the reduced amount of court coverage needed as well as a lower threshold for skill to compete. Experienced tennis players should find the game easy to learn. Most well accomplished tennis players tend to say that once their mobility goes away is when they'll take up Pickleball.
Court Dimensions - Having your own court should be considerably less expensive since the court dimensions are considerably less than a tennis court.
What about Padel?
Another racket sport that has been around since 1969 is Padel. It is played similarly to tennis but on a smaller court.
Here is some information on the sport of Padel.
Rules of Padel
What will happen to Tennis?
Tennis isn't going away but it now has some follow on sports that may lend to the overall popularity of racket sports. If this occurs then it is likely so much better for tennis players in general though it may cause problems between tennis players and pickleballers when it comes to court space. It's happened in other sports so it could easily happen here. The smart thing would be for communities to make sure there are enough of both kinds of courts to serve the public demand. One way of doing that would be to build lots of tennis courts and add pickleball lines on at least half of the tennis courts though I can see where eventually it will be done with all of them.