1st rd picks are important, and you don't want to miss too often.
I'd claim missing is more important than hitting a generational player because if you miss consistently, you're missing out on 2-3 players who should be 3+ WAR starters, whereas you might hit a generational player once every couple decades. So it's a random event (and usually a no-brainer decision), whereas finding starters in the first round is more a reflection of good scouting and under your control.
Going back 30 years, 1987 Griffey (84), 1990 Chipper Jones (87), 1993 Alex Rodriquez (118), 2000 Adrian Gonzalez (43), 2001 Joe Mauer (53), no one else over 40 WAR.
Of the most recent guys, Harper has one great season out of 6, and 2 good ones, while he's entering his peak years, there's no sense that he's doing more than holding steady, I don't think he qualifies as a generational player.
Strasburg can be great when he's on, but do you want to bet on his durability. He's only had one 200+ IP season.
Correa is off to a good start, but way too early to know if he's the next Mike Trout.
Most #1 draft picks are good players who may at best give you a 5-6 year stretch of 4-6 WAR seasons. Valuable but not franchise altering. And about 1/3 are basically flops.
However, the majority of starters come out of the first round, so missing on good players can add up after a few drafts.
I'd also claim that 1st rd picks are more important than top LA signings (other than Cubans a few years ago, because the top Cubans were old enough to be sure things).
Talent is spread more evenly in LA signings than in the amateur draft (because 16 year olds are harder to project than 18 year old HS players/21 year old college players).
But teams that have a real edge find other talent consistently. Including raiding other teams' prospects through trades.
Again, scouting is important, not only other team's talent, but your own talent (who to keep, who to deal).
PS; Looking at the past few drafts, only a couple guys from the 2015 draft are in the majors, mostly mid-sesaon call ups, and Swanson is struggling, A few hits from 2014 - all college players.
So figure 3 years miniimum, maybe 4 for any impact from the 2018 draft, depending on whether we draft HS or college.