So what technology do we need to automate away personal trainer jobs, and when do you think the purge will be complete?

Are you really trying to entrap me into turning my observation about the present into a prediction about the future that will be wrong?

Automation of personal trainers is already ongoing. People have been more successful on automating the "motivational" aspect of the job (probably because this one isn't regulated). But there are lots tentative initiatives to automate the "teaching and correcting" aspect too, from video games to in-lab research.

I just think predictions are worthless if you don't put any kind of time horizon on them and don't explain why existing trends indicate otherwise.

As you said, we have seen a proliferation of devices, equipment and information that make it easier than ever for people to form a fitness plan and measure progress. We have also, at the same time, seen a large increase in the number of personal trainers in employment. This would suggest that technological progress in this sector is complimenting/augmenting, not replacing, the work of humans.

As their job gets automated and more productive, we'll probably see more employment of personal trainers. At least for a while. There are many people that would like to hire one, but won't because it's too expensive - as it gets cheaper, more get hired.

Programmers are on this same stage of the cycle. But I expect we to last there much more than personal trainers.

If you went back 40 years, you'd probably find people saying accountancy was a dying profession, that it was only a matter of time before their function is entirely carried out by computers. And indeed we have software for probably every accountancy task, just as people imagined back then. We've had it for years. We still have accountants.

People are willing to pay to have someone shield them from the complexity, the diligence, and the organizational effort behind everyday endeavors. That's not going to change.

Also, automation's track record is incredibly overrated. It seems to only work in situations with exquisitely controlled conditions, like assembly lines. People bring up super-market check-outs, but this only works because the customer does all the physical work themselves. The bridge between automation and complexity is intelligence... and we're nowhere even remotely close to having AGI technology.