Where my analogy and your idea of “punching them in the face pretty hard from day 1” falls flat is that you can not eventually, successfully box or spar without getting punched at least a little. But you can! play RPGs and even an OSR-style game without needing to make it super lethal and stick to the rules and their consequences 100% time, no matter what players experience level is or what the situation is. Hell, even on this forum, there is Chiquitafajita, she is super interested in / busy using OSR, but is looking for a game that is very low on darkness and combat and very cosy, I think that is a totally viable and possible way to do Old School gaming. High lethality is only one of five-ish components that make up an Old School experience for me.
(New) RPGers can absolutely learn how to play the game and have a ton of fun and be extremely challenged without the GM needing to be 100% strict from the get go. After a few sessions (when the newb players know at least the basic rules!) you can of course make a breaking point and be like: “From now on stuff gets real, we are going to play more strict and your PC might die, often even. So let’s all make another back-up character and remember to play more carefully.” You gotta make them want to come back! And perhaps ramp up the difficulty based on their experience. Otherwise you run the risk that OSR becomes what old D&D originally was accused of: only for nerdy people, really hard to learn and by definition dark or violent. I don’t agree with that assessment of D&D, but I do want to reassure new people of that by showing them.
Even in boxing, there is no! reputable boxing gym in the world who would do that! -> Punch a total newb hard in the face during the first lesson ever. Hell, most won’t let you spar or even in the ring until you have a decent amount of lessons = know the rules/basics! If you can not defend yourself or know the rules, it is not fair to apply the rules to the max or to make you play (box) at full tilt with people that will demolish you. Then there is also weight classes, innate aptitude/athleticism etc, that a good trainer would 100% consider.
Hell, some of the most respected boxers of all time and some current champions are so respected because they get hit extremely little, Mayweather, Fury. Never mind fairly recent knowledge on CTE/brain damage, most professional fighters spar at 60% of power or less, with headgear, even with extremely experienced opponents.
To bring it back to RPGs and even OSR a little more, some people box or play RPGs to relax, or to get fit / a bit better at it, some people are going for hyper realism and rules as written, all the time. Neither is “wrong”. But I do think if you don’t tailor your game a little! to the experience of your group (whether that is using way too many rules and overpowering them because 5e book says to do that) or making them feel like they fail or suck at DnD by being super harsh, 100% of the time from the first second they play,… I think in both cases it might miss the point. The point is to have fun. When the style of play or rules get in the way, you throw it out the window. Even Gygax was a huge proponent of that, it is in the DMG, nonwithstanding how harsh he liked to play.
And he played super harsh with experienced roleplayers and wargamers or people that knew exactly! what they were getting into because Gygax had that reputation. Other than co-inventing RPGs it was what he was and is most famous for.
Even in certain OSR systems, with inspiration or action points or advantage, those are all really arbitrary decisions the GM makes. When you give them, why, how good a player needs to describe things or how good the plan must be before you give them those boons,… it is not hard and fast and set in stone at all.
I might give a total newb player advantage or more likely inspiration if she grasps and successfully uses a bit complex mechanic for the first time ever and proceeds to roleplay that action rather well, but an experienced player I would expect a heck of lot more off, before I gave them advantage or inspiration.
At the end of the day, if your players are having a lot of fun and feel very challenged and keep wanting to play with you, you are doing it right! I just always try to remember that what is very challenging and fun for one (old hand), is likely nigh on impossible and even severely demotivating for another (newb). But yeh it does appear we might have to agree to disagree. Everyone has their own experiences and preferences and style.