there really is no definite answer to whether or not a lag cap should be in place. lemme apply some of my college knowledge of ethical theories.
a popular one would be utilitarianism which maximizes the benefit for the largest number of people. if we followed this and set, say, a 450ms lag cap to sparring ONLY (which is what this thread is about), then people dont have to deal with this insane "VPN" stuff which i never really bothered to understand. also, it doesnt really matter what region you are from if the lag cap is at 450ms, no player from any area has ever said they average above even 300ms. so back to the theory, many players would be happy with this kind of lag cap in place, and the only players who would actually get hurt are those players who are abusing their lag.
however, this theory gets some competition in the "ethics of care." this says basically to protect those who are close to you. you can see this used in the argument from the alumni side (points out that only fulgore thallen and shannon are really saying "lagging isnt cheating"). some players are close friends to those who are abusing their lag to obtain easier wins, so of course these people would be against the idea of a lag cap in spar, no matter how logical it is.
i would apply some more theories but i am getting distracted and talking about my plans for christmas presents so ill just end it there. youre welcome for the free education. there will probably never be a lag cap though, as certain corrupt people get really pissy about it since it removes their advantage.