Yes, I think there’s a very good chance he would have. As Sanket Jariwala points out, Lars essentially created thrash-metal drumming. By the time of that bus crash, he’d fully demonstrated himself as a premier drummer in that genre and metal at large. He also would’ve had none of the “baggage” he gained later on with his musical choices, self-admitted laziness and lack of practice, and extra-curricular controversies (e.g., Napster) that have, fairly or unfairly, impacted how people see him.
The only hesitance I have in a solid “yes” is that we wouldn’t have had his drumming on …And Justice For All. Some of his most legendary work (e.g., “Dyers Eve”) is on this album. Apart from this, assuming Metallica even continued after his death (a big question in itself), could they have created such a masterpiece with another drummer? Hard to say, but with the pool of drummers available at that time, it’s unlikely — a few others were capable, but Lars and Hetfield were so incredibly tight. You can’t just replicate that by bringing in another drummer, no matter how capable.
Metallica’s popularity and influence grew in magnitudes with each album from 1986 to 1988 to 1991. If this had been cut off in 1986, Metallica would’ve been far less known than they have ended up. How much would this have affected Lars’s chances of being a legend? It’s like asking whether John Bonham would’ve been legendary if he’d died in 1970 versus 1980. On the other hand, Bonham only got better, whereas Lars hasn’t developed that much as a musician in the past 30 years, especially relative to it being 30 years.
Would Cliff Burton be seen as slow and in need of replacing?
Barring some ongoing or life-changing medical, health, or personal problem over the years, I don’t think so. Burton was far more dedicated to his craft and studied more than Lars did. Remember, in some ways, Lars was an accidental drummer and rock star. He was on the path to becoming a professional tennis player, and didn’t even start playing drums until he was a teenager — just a few years before starting Metallica. He never seriously studied music.
Conversely, although Burton didn’t take up the bass until he was 13, he’d already learned and studied piano and classical music. His stated influences were far more diverse than Lars’s, which is typically indicative of someone who studies their craft. And finally, he reportedly practiced several hours a day right up to his death. So, this background and trajectory demonstrates to me someone more like the Rush drummer Neil Peart — who, at the age of 42 and already a drumming legend, took it upon himself to hire a world-famous coach (Freddie Gruber) to completely re-learn his technique and approach to percussion.
I see Cliff Burton as far more likely to have continued his own development and never slowed down. In fact, I’d go so far as to say he might’ve outgrown Metallica at some point and moved deeper into progressive metal, possibly even starting his own band. Seems a good chance he would’ve done this instead of — or as a result of — the “black album” if he had no say in the band’s choice. I have a hard time imagining him meshing with the style and sense of the album’s producer, Bob Rock. Maybe, just maybe, Burton would’ve seen it as an interesting musical exploration, but I doubt it. I think he would’ve been far more attracted to what bands like Tool and Dream Theater were doing at the time.