The last point you made is why Garza might be tempted to leave. Carsen Edwards (projected in the 1st round last year) didn't have much else to do at Purdue. 2nd team All-American, 1st team All-B1G. He lasted to #33 (3rd pick of 2nd round). Cassius Winston was the B1G Player of the Year last year - has he improved anything this year (including his draft position)? Or, is he just a year older? Stats are similar from last year - and yes, I understand his brother's death has impacted the situation.
I believe that's what Tyler Cook felt last year as well. Sometimes (like some football players), it is about getting the clock to start ticking. An extra year of college is not going to help anything (in some cases) and the player just feels he is ready to go - and get paid.
It will be interesting to see what Luka thinks going forward.