Unwritten rules are literally the worst way to govern materially relevant aspects of a game. And yes, running sharp singles between the wickets is materially relevant, especially in the limited overs format.
The solution you've offered, of having the third umpire decide non-striker run infringements and penalizing runs, is also inadequate. All of these solutions, are putting the onus on everyone involved to be complicit in creating a subjective gray area of "acceptable backing up" that is preserved through very soft ( warnings, run penalty, or even ignoring it cuz we're a good sport innit ) means, instead of the perceived draconic consequence of a run out at the non-striker end.
Why does a certain subset of the cricketing word culture want to bend over backward to promote this environment in which this gray area can be perpetuated? Is it because western cricket cultures are permeated with and promote microaggressions, pushing the envelope, and just a general underhandedness to win at all cost? Actually the reason doesn't ****ing matter. But it should be obvious to anyone with 2 brain cells to rub together, that the play environment in which the non strike batsman waits to see daylight between the bowlers hand and the ball before leaving his crease out of a back of the mind knowledge he can and will be run out vs the one in which there's doubt as to who can get away with exactly how much will have a huge difference between the two in the number of pointless arguments, tears and handbags.
There should be no reason to bring umpire subjectivity, or even an unwritten rule into play to legislate an aspect of the game that can be very easily settled through a black and white, letter of the law (which thankfully has been updated to make very clear that running out the non striker is letter of the law).
If you want to watch a sport where you can always find some complaint about how umpires are interpreting any number of inherently subjective rules, then you're spoiled for choice between football, basketball, rugby, etc. Go watch one of those, while the rest of us move forward with a fair, explicit, and practical rule set for cricket.
