He would be wrong, it's a stat, just a stat, not good, not bad, just a stat. Just because other stats have been around as long and tell you more does not make it a garbage stat, its just stupid to disqualify a stat using that criteria.
Let me try it, HRs is junk stat, it does not tell you if it was hit off a slider, fastball or knuckle, it does not tell you if the pitcher was tipping his pitches, it does not tell you if it was hit into the wind or against the wind, it does not tell you if it was with base runners on or bags empty. It does not tell you the Humidity level in the Air. It does not tell you if it was a short porch or straight away. It does not tell you the count.
I've been playing or coaching ball for over 45 years. Never , have I ever heard a person call RBI's a garbage stat until today. I've played with and against players who went to the MLB, I've had beers with former MLBers and never ever has anyone after knocking in a few RBI's say yeah I might have had a few RBI's but its a junk stat.
RBIs are a bad stat because they have little evaluative or predictive value. They don't truly tell you much about what the player did considering there's little that separates a hit that garners RBIs vs one that doesn't other than things outside the batter's control. On a game-by-game basis you're better off just qualitatively evaluating someone's plate appearances or using straight up hits or walks or whatnot. On a seasonal basis you're going to get more out of HR, OBP/OPS, wRC+ or the like.
Small sample issues, but let's just look at the year-to-date league results.
Mike Trout is... Mike Trout. The best hitter in baseball. Vlad is in or near the top 5 in most decent broadly evaluative offensive stats at the moment.
Where are they on the RBI leaderboard? In the 40s, tied with several other players with 12 apiece. Kyle Tucker in Houston, who's currently batting a sterling .181 has more RBIs than either of them. Travis Shaw has more. Trey Mancini has more and he's struggling to stay above the Mendoza line and is overall hitting like a below-average player.
They also don't really tell you anything about a player that is concretely useful moving forward. Sure, most of the time high RBI totals correlate to "good player". But they can also go way out of whack with little surface explanation. It's how you get Joe Carter topping 100 RBI in 1997 while also being pretty much the worst regular hitter on the team outside of Benito "I'm a catcher so I don't really count" Santiago. Or 100+ RBI seasons from luminaries like Adam Lind, and Tony Batista.
I think people say RBIs are a junk stat because if you're less strident about it, that becomes an invitation to 'yeah but' excuse its shortcomings. The reality of RBIs is that while having racked up a lot of RBIs in a season is cool, there are better stats to look at when you want to answer the question of how well a guy did in a season. And yes, HRs might be one of them if we're presuming that the choice is "you get
one stat and that's it." It's obviously not a
great stat for overall offensive prowess, but it's more likely to be a reliable one than RBIs. It's easy to get into whatabout arguments like "well HRs are junk if you don't filter for pitch type or park dimensions or opposing pitcher fly ball rates or whatever" but the point isn't that RBIs have potentially obscuring drill-down splits and quirks. It's that holding all other things equal they're more likely to be faulty than another stat like HRs.
If you can acknowledge that pitcher wins are too noisy and messy to be useful as an evaluative tool for pitchers, the case against RBIs is largely the same when it comes to hitters.