“You” is used ambiguously in this post, not aimed at one particular person or group of people.
The whole reason someone may choose to use “should” is to stop it from being aggressive. You “should” learn PLL around sub-20 and OLL around sub-15. It’s not a universal truth. It’s not absolutely required to improve at those points or entirely detrimental if you learn it before then. But for most people most of the time, those are good general benchmarks. For some, it may be required to get sub-30, or maybe they like learning algs and learn those alg sets super early. For others, they may put off learning algs for as long as possible. When giving advice to someone who has only provided generic information about them (“I sub 20 how be sub 15?”), using this principle of “most people most of the time” is very helpful. Better advice can be given if and when there’s more specifics of the individual’s circumstances. I don’t think there’s a need to imply aggressiveness or judgement.
Cubing is an extremely individual activity. If you’re worried about self-confidence and think that it can be significantly negatively affected by someone saying that PLL is usually learned around sub-20, then your time will be much better spent on working to improve your self-confidence and self-worth (which is an extremely valuable thing) rather than attempting to baby-proof your life (which is an ultimately impossible task). Cubing is the least likely thing in your daily life to hurt your self-confidence, and it’s much more important to have a stronger self-image (which can be difficult but is important). If you’re struggling with “you should be able to” affecting your self-confidence, you’re just as likely to be discouraged by WR holders, other’s fast improvement, etc. I don’t think there’s anything wrong with people providing benchmarks and saying what is “average”, or best for most people most of the time.
As an extremely competitive person by nature, I’ve had to learn that in my life, when I’m competing in the field of the entire world (such as cubing), I suck. I will probably never be the best or even world class at anything with a meaningful amount of people competing. The solution isn’t to only engage in activities and competitions that I will probably win, the solution is to learn the difficult but true lesson that I need to focus on my own individual improvement, and maybe compete with someone similar to me on an individual level, but not be worried or upset if I don’t win my competition consisting of the 100+ best cubers within driving distance of me, or feel bad that Yiheng Wang is actively breaking the laws of physics to accomplish his feats while I’m still “only” sub-15 (the idea of fast vs slow is also relative and adjacent to this conversation but I won’t get into it now).
I think the following interaction between Mike Hughey and I demonstrates the usefulness and downsides of the “most people most of the time” principle:
[In a thread titled “why cant i do lookahead”]