Firstly, I just want to say: yes, we'd love a 50/40/90 guy. Sadly, so would every other team in the NBA and there's only about 3 of them, two of which spent all of last year on the shelf with injury, so that's a hope that will probably never materialise sadly.
I agree with what you say mostly: Lonzo simply cannot be a viable shooting guard right now. Wing is a little different though, because if you play him as a forward on offense, then all he really has to do is spot up. It would be nice if he could attack closeouts as well - which he can't - but if he's taking 10 shots a game and 8 of them are 3s, then it's the 3pt% that matters more than anything else. Shooting 38% on 8 a game would be functional. The FT% is still a huge issue because it means he's hard to close games with, especially when we have other, more versatile players like Zion who can score at the rim, who can drive and create for themselves, who can collapse defenses to pass, and who therefore obviously get priority over Lonzo on the floor.
Here's how bad Lonzo is as a scorer overall. Last year was the single best shooting season of his career by a mile: he set career highs in 3pt% and FG% while increasing his 3pt volume by a large amount. The result of this was that he set a 51%TS mark: the first time in his career that he hit over 50% TS.
League average is 57%TS. So it was the best efficiency season of his career by a landslide and he was still quite significantly under league average overall.