That's the gamble. While it would be great for memories to come back for your senior year, if you are considered amongst the top talents in the draft, the risk of a career-ending injury in the day and age of multi-million dollar contracts for rookies just isn't worth it. Now, if he was, say, projected as a mid-rounder or later, you could argue that the risk is worth it (both for the memories and to potentially improve the draft stock). It all depends upon individual priorities and what level of risk you are willing to accept.
I think the mid-round guy is risking more than the likely first-rounder. If Ben Niemann had been injured in the Pinstripe bowl, he likely would have never gotten a shot at the NFL (the Drew Ott example pretty much proves this is true). As it is, he will be a multi-year pro who will likely make several million.
The examples that everyone likes to use of players who lost big are Jaylon Smith and Jake Butt. They both had multi-million dollar insurance policies to cover them in the event they could never play pro football, as well as in the case that their draft stock dropped. So they got hurt, their insurance compensated them, and they both were drafted anyway based upon their prodigious talent. Smith will end up earning tens of millions in his career. Butt signed a 4 year, $2.7 million contract. Not sure how much Butt will actually get (he tore his ACL a 3rd time last year, after also tearing ACLs during his Freshman year at Michigan and during his senior bowl game).
So Smith will end up makings tens of millions, Butt will end up making at least a million. And those are pretty much worst case scenarios. Had Butt not gotten hurt, maybe he goes 1st round and gets $5-10 million guaranteed before tearing his ACL as a pro.
So who is risking more, Butt, who could have gotten $10 million, but was pretty much guaranteed at least $1 million no matter what? Or Niemann, who will end up making several million, and would have gotten bupkus had he gotten hurt? I would say Reynolds and Render were in a very similar boat this year, although it is looking less likely that Render gets drafted following pedestrian pro day numbers.