I am not convinced one way or another, but my knee-jerk reaction is somewhat against it. Some thoughts/questions...
Since they tend to declare in December/January, signing day goes to February, but the draft isn't until April, what happens if an undrafted kid wants to come back to his school but there is no longer space (because they signed his replacement via HS or the portal)? Would they have to come back to their same school, or immediately enter the transfer portal? Will these kids still count against the 85 limit? As an IC if they come back to a team other than the team they left? Does this incentivize marginal draft picks to declare for the draft just to see what happens? Could be a risk for those kids who get bad advise but end up drafted in the 6th or 7th round (where nothing is guaranteed) when they could have come back, made an impact, and greatly improved his stock for the coming year (and maybe made some $$$ w/ NIL). There's a lot that would need to be worked out to make this viable and (somewhat) fair to all parties.
On the other hand, keeping these kids out in the cold for making a bad business decision is a harsh penalty. I have always appreciated that UM allows them to come back to finish their degree for free, but obviously none of us want to see any former UM players who contributed to the program tossed aside for making a bad decision. Also, this would obviously only apply to kids who leave with at least a year of eligibility left, so maybe it wouldn't be such a strong incentive to declare afterall?
Definitely interesting, but a lot to consider.