true enough but what else have you got then besides old-school conventional wisdom, the thing from whose errors and prejudices we are trying to inoculate ourselves in the analytics era? If a guy holds his own for 120 PA against LHP and in particular has a reasonable K rate - a number which usually does stabilize pretty fast, where's the strength in the argument against letting that keep playing out? It comes down to someone saying "well I've been managing baseball for X decades and I believe most guys will crash back to earth"
But any particular guy is not necessarily "most guys", so if all you have is a general prejudice, as the Dude would say: "Well, that's like..just your opinion man"
this also an area where you would think tech should start making a difference. The reason given that LHH have worse platoon splits on average then RHH is that RHH have no choice but to see a lot of RHP. But with Trajekt (et al), a LHH who wants to work at it can practice against good LHP as much as he wants. We'll see if that has any effect across the league over time.