It is economically untenable for the overwhelming majority of university and college presidents/athletic directors.
The playoffs would add 15 post-season games to the college schedule, the effect of which would be to kill the current bowl system. Now there are those Utopians who dream you can incorporate the current bowl structure into a playoff system; the problem with that argument is that it's not the bowl structure that needs to be preserved, it's the bowl games themselves.
I can see SMU President Gerald R. Turner having fits about such a playoff system. SMU head football coach June Jones is not, right now, striving to make the Mustangs one of the top 16 teams in the country that could make a playoff; he just wants to get them bowl eligible and into a bowl game. That would be a huge plus for this team. Do you think he's gong to favor a plan that kills his chances of doing that? That will really help his recruiting efforts.
You could try to keep many of the current bowl games going along with the playoff games, but that would mean those bowl games are to college football what the post-season NIT is to college basketball: completely irrelevant. And where would the money come from for these bowl games with television revenues being diverted solely to the playoffs?
Here is the only way I can see a playoff plan like SI's working: Allow all the teams that don't make the playoffs the opportunity to schedule two additional games of their own that same season. Actually, I have been mulling various scenarios for this around in my head and you could really have fun and spark regional interest in pair of games played on a home-and-home basis with the outcome decided by total points of the two games.
Plus these extra two games would answer the objections of the teams who don't make the playoffs by giving them the opportunity to generate revenue; obviously not as much as a playoff team would make but at least it's directly generated revenue that would be better than the totally unsatisfactory "spread-the-wealth-among-conference-teams" that current playoff proponents are suggesting would satisfy those schools not among the Top 16.
But I don't think those proposing a college football playoff care one whit about the 100+ teams that don't make the final 16, only the handful that do. And as long as that's the case, you're never going to get a playoff.
I will continue to update SI's ongoing bracket as the year progresses. I mean, what harm can that do and, besides, it makes for a nice diversion from reality.