If you look at the top 30 players in wRC+ in 2018, it took me to the 24th ranked player (Baez) to find someone with a BB% below 9%, and he had a BABIP close to .350.
If you focus just on players who had a 120 wRC+ or above (54 players total), the players who had BB%'s at or below 7% (pretty much Grichuk's ceiling to this point) were Baez (4.5), Andujar (4.1), Gennett (6.6), Piscotty (6.9), and Cron (6.6).
WHat you are saying is not wrong, a 120 wrc+ is a 120 wrc+ regardless of how it happens, but look at the probability of Grichuk being one of those players (or better) with his BB%. It is a detriment to his game.
That's not meant to be a dig at Grichuk, he's not even close to one of the team's issues right now, but you're fooling yourself if you think a lineup full of players who walk at 5% clip is not going to lead to middling offensive performance overall. It's hard to be good offensively with that type of walk rate.