Technology in terms of the implants and the surgery is changing so fast it's hard to predict. From what we can see so far, it looks like joint replacement will be as common for the Baby Boomers in the 21st century as taking out the tonsils was in the 20th century.
Another trend might be getting joint implants at a younger age. Partial replacements will be a popular way to prevent uneven wear and tear from continuing once the joint starts to break down. Scientists are also looking for ways to regenerate joint tissue to avoid replacement altogether. Stem cells may play a key role in that research.
Hopefully the next generation will remain active with lower rates of obesity and arthritis. This might help them avoid joint problems in the older decades. Better care of joint injuries during the younger years will also make a difference. For example in the knee, cartilage tears meant removing the entire meniscus. This led to more joint damage later. With repairs of these injuries rather than removal, the future looks much better for injured athletes. They may be able to avoid joint replacements.