Training to failure can be an effective way to increase strength. (Google Ken Leistner, Kim Wood and Arthur Jones, and read some of their writings if you have any doubts in that regard.) However, it is not the only way to get stronger or, in my opinion, potentially even the best way to get stronger. Some incredibly powerful men (for example, Ed Coan and Kirk Karwoski) would commonly train for several months leading into a powerlifting meet without ever missing a rep, rarely ever approaching failure.
It should also be noted that most people (myself included), even when attempting to train to failure, usually don't come even close to approaching real failure. Under the tutelage of a prior coach who was particularly skilled in "motivating" his lifters to push deeper and deeper into their pain threshold, I got the point where I could push a set of high-rep (20+) squats or deadlifts in excess of 300 pounds to the point that I was shaking and vomiting afterward. I did get stronger during that period, but I can tell you that my motivation to train diminished over a period of several weeks. It is hard to stay motivated when you know you're going to be completely wrecked in a few minutes and that you're going to repeat that experience a few times per week for months on end. (I now compete in time trials, so it's not as if I try to avoid discomfort.)
In my experience, the most important component of strength training is consistency, based on showing up and adding small, incremental gains for long periods. Most people don't have the patience for that, but there it is. Consistency requires, among other things, (1) staying injury-free, and (2) staying motivated to train, day after day, week after week, year after year. I think the number of people who enjoy training consistently to failure are in the minority, so maintaining motivation to train becomes a consideration. In addition, when I go really deep and push to approach failure, at some point, the pain and discomfort become so intense that I start to get a little, for lack of a better term, "loopy." Unless I have someone nearby who is watching my form and making corrections, it is really easy for good lifting technique to break down, and that's when injuries happen.
To be clear, we may just be defining the term failure differently. If the point is simply that you need to, at least part of the time, train hard and push your limits somewhat, I don't think too many people will disagree.
Last edited by SkepticCyclist; 08-13-19 at 11:34 AM.