I feel like with combat (and i suppose politics if you're under pressure) specifically, the mentality part of it is such a big part of your performance in a way that doesn't really apply to something like programming. World's best crackshot is going underperform if he hasn't cultivated the proper mindset for the battlefield, which you can't really get outside of the real deal (or, I suppose in mfd, very specifically created genjutsu)
Like with this example, I would look at it the other way around. Street brawlers who are accustomed to real scraps and the fear of actual danger are going to be performing closer to their best than a guy who's never thrown a punch outside of a dojo getting mugged in the alley.
That's actually how the system works! You will still have an xp rate, it just decreases slightly over time if you don't get field experience often enough. This isn't a huge deal unless you are minmaxers like us who fret over the smallest decrease in xp income.
no comment on testing in principle but I can agree with any criticism of the US education system
Cultivating the proper mindset for the battlefield or underperforming in battle because you're unblooded is something entirely separate from require deadly combat to improve, and is a problem that goes away once one's a combat veteran anyway.
Incidentally with all the combat that has happened over the course of the quest, combat veterans is something the members of team uplift definitely classify as.
"Let the Bodies Hit the Floor" for instance defintely showed us that Hazou does not at all suffer from performance issues in combat.
Also calling a xp rate decrease of up to 90% a slight decrease is positively ludicrous.