A couple of things.
If the kid thinks he can manipulate the system to suit his own ends as a senior, he should fire his agent. A team will draft him low, keep the rights and the kid will not sign or play. Where does that get him? (Guys have tried this in the recent past to no success - even going so far as to play indy ball. The moral of the story is the dog is MLB and the players are the tail; the tail is simply along for the ride. )
But, all is not lost.
The tippy top senior pitcher(s) will indeed be selected in the first 3 rounds - if he will discount his bonus. BUT the discount is less than a 9th or 10th round senior pick. (E.G., back in 2014 a Maryland senior - Jake Stennet - got 1 mil). These are the mid-90 guys who can also pitch. The next level of senior pitchers get drafted 9th or 10th round and discount about 90% off slot. These guys have demonstrated senior year dominence combined with showing an ability to pitch (i.e., its not all projection); also these guys have some degree of velo (sitting 90 for a LHP). The last level are senior pitchers who had a great senior year AND display a potential MLB tool. These guys simply get a chance to play proball - and if they deserve it, move up.
(if a senior pitcher holds out for slot, NO TEAM WILL DRAFT HIM ABSENT A PREDRAFT AGREED NUMBER. If a team slot drafts a senior who doesnt sign, someone gets fired.)
Age is the enemy of every proplayer. Decisions flow from that. While there are exceptions, those are extremely rare, and take a 10% success rate, far lower (e.g., Appell).
It is a red flag if a guy isnt willing to sell his soul for his passion - a chance to play proball. Scouts have recounted to me stories of their top draftees who didnt give a hoot for the signing bonus - their thirst for proball was dominating their thoughts and plans. Scouts often refer to huge signing bonuses as "failure bonuses" (their thought is that a first rounder's ultimate career earnings will dwarf the signing bonus).
So, for that kid who chooses his senior year (not because his junior year offer was perceived as low, but because he wanted to stay in college) over proball the risk is all his; no team will view him as a savior - because the kid's choice proved proball wasnt his highest priority. No one makes it unless he is all in, all the time, waking and sleeping, 24/7 365.