Why are so many people discussing the evidence here?
If one seriously believes the BMJ would let a questionable study be published, think again. I'm not saying everything that is published in high quality peer reviewed journals is absolutely true, but it is subject to so much scrutiny that is unlikely to have an evident flaw in the reasoning, or at least some that is not properly mentioned in the article or letters to the editor.
Which leads us back to why are so many here angry at the conclusion - because many answer seem emotionally charged.
Among the tools offered to you to try to increase your lifespan, and especially the "high quality" years, is physical effort.
There is even a very positive message there - you don't have to be in a perfect physical form, or do sports, to get the gains- muscle mass alone is enough.
If you have had health problem, say broken bones, reduce mobility, pain, whatever, you can still get some of the positive advantages of muscle mass with weight training - which can be done at home, in a gym, anywhere.
You may not get as much benefits as somebody fully healthy (ex: if you hip is not working, it will be a problem to train both legs, etc.) but it is still better than nothing!!!
Exercice, as in improving the muscle mass, is well known to have positive health effects. If one does not exercise, the blame is not to be put on the lack of time, but rather on the lack of proper prioritization.