If taking the best player available is always the best option, then why do people consistently underpay their top guy to get more guys later? An A prospect and a 3 C prospects is only better than 3 Bs and a C if the A is a Harper, Strasbourg, Cole, rendon, torkelson, etc.
Especially this year where there is a strong crop of like 8 guys (rocker, Leiter, Mayer, lawler, jobe, Watson, house, Davis) if one of those guys gets to us, then snap them up. Otherwise, a college arm or bat who may not be a star (but likely a major leaguer in some capacity) who will take an under slot bonus could be the move.
Anybody who is a sure enough major leaguer to fit into the scenario you're thinking (low bust risk) is not taking an under-slot bonus unless he's a relief-only prospect or a no-hit catcher that likely profiles as a backup. There are precious few scenarios in which either is worth a first round pick.
Sure, you can take a college senior in the first round if you want to tamp down the bonus, but then the high-upside HS guy you want later down better still be there and you better be able to get him away from Vanderbilt or whatever. And at that point he damn well better be at least as good as the high-floor guy.
The draft isn't the crapshoot it used to be in the first and second rounds. Analytics has changed a lot. Look at the first round of a draft as recent as 2018. Instead of 2/3 of guys making the majors at any point and any level of success and a significant number of total flameouts, you have this: every single college guy drafted down through Competitive Balance A (except Kyler Murray, for obvious reasons) is at least in AAA, and most have MLB experience. And there have only been two drafts since that one!
These guys have agents. The agents are not statistically illiterate, or at least employ people who aren't. The travel-ball circuit is so sickeningly ceaseless and video so ubiquitous that nobody is going on word of mouth or thin scouting opinions. College ball is on TV now, and everything is on Youtube. The guys who are good know they're good, and there's little incentive for anyone to play along with slot-jockeying. It's not a crapshoot anymore, at least not at all to the same degree.
Unless, that is, you start freaking around with it and doing stupid crap like drafting high school pitchers or trying to lowball guys.