I think it’s inevitable at some point, and quite obvious, that you’ll have to increase volume.
Once you are training to failure your intensity is maxed out (let’s assume your exercise selection and form is appropriate for the goal). Couple that with the fact that strength only gets so high and there’s only so many ways you can make a top set appropriately intense.
Let’s take a trainee who has been at it for 5 years, gotten all the newb gains and has made respectable strength gains.
Let’s say they are squatting 315 (or 405 or whatever really, pick a number) and can get 12 reps to form failure ad your top set and this is a number they have worked up to over the years and the progress is slowing down… they PROBABLY aren’t going to add significant weight to the bar over the next few weeks, and they probably aren’t going to add significant reps in our one set to failure program.
So to further adapt the body you almost HAVE to find ways to increase volume… maybe that’s also by doing an intensity technique like a drop set, or extended set, or super set, but all of those are still adding volume. The “easiest” way Is to simply add another set or another exercise for a set. Of course you can only do that for so long too.
For me personally, volume training my legs creates too much soreness and immobility now that I have a kid so I stopped it, but I don’t think you are going to go wrong with 3-4 sets of something.