I agree with this idea but if you work out badly enough you can seriously injure yourself.
But plenty of people have permanently damaged their knees by consistently running without getting advice on how best to do it.
You don't need a personal trainer for that. But asking someone "to tell me how to run" ain't a bad idea, even if it's a question on a running board on the Internet.
The people who read running boards all the time roll their eyes at stuff like this. Saying that running will hurt your knees is the running equivalent of "The GPL will infect your codebase." It's not true, but it sounds scary.
Usually, if you ask runners how to run, they will say something like, "It's not that complicated. Run more miles, slowly."
For example: https://www.reddit.com/r/running/comments/wi8zq/want_an_easy...
OTOH, "Runner's Knee" does seem to be a serious condition affecting a lot of people (much like "Fencer's Knee", which I'm more familiar with). From the same subreddit:
https://www.reddit.com/r/running/comments/3hx43k/how_i_cured... https://www.reddit.com/r/running/comments/26jaax/3_years_ago... https://www.reddit.com/r/running/comments/1gdw5l/can_i_fix_m...
And a number of people, when discussing the problem, reference bad technique as a causative factor.
However, you're absolutely right that it seems there's little evidence for permanent damage. I found this thread particularly interesting:
https://www.reddit.com/r/running/comments/1fehx2/wife_trying...
I trained with one on the Starting Strength lifts for 4 months. Made crazy gains, but I ended up stopping after an injury. I was not listening to my body the way I would have if I hadn't had the trainer.
I'm starting Crossfit now, and my number one goal is zero injuries. I like that it's more of a class format than it is a one on one training format. I can go at my own pace.
https://medium.com/@ericrobertson/crossfits-dirty-little-sec...
I personally would prefer to just lift. But unless you're pretty lucky, or are willing to invest the time and expense, you won't have access to a gym with decent equipment. Since convenience is a huge factor in lifestyle choices like working out, you need to choose amongst the activities that are close to you. Once I graduated from cardio at Planet Fitness, the only place to go really is Crossfit.