I think this is best answered as two questions.
As a cardio regime I would send you to the very interesting book, “The Science of Yoga” by william Broad. In one of the chapters he does an excellent job of parsing the history of research into aerobic benefits of yoga. The bottom line is that for real cardio benefits yoga, even hot yoga, is not going to be the best option. It is a bit complicated to give you the whole story, but I really recommend the book, and you can see what you think.
However, fitness is a multidimensional concept. The health benefits of yoga are well documented. As well, depending on the form you practice, there are benefits to various dimensions of fitness: flexibility, strength, balance, etc. to be had.
And as a long term physical regimen I would ask you to google pictures of Indra Devi, Sri K Patthabi Jois, or BKS Iyengar. There are teachers out there in their 90s still able to balance their body weight on their arms. I have done yoga on and off since I was 9, and I am in my mid 50s and absolutely believe in the practice.
Of course, there are huge differences in the type of practice one does. Some varieties are more injury prone, some provide more mind body benefit, some more strength benefit, and so on.
I think the best thing is to consider fitness as neither linear nor monolithic. As Dean Ornish showed in his work with the multifactorial treatment of heart disease it is useful to have more than one way of engaging. That is one of the joys of fitness…..