Michigan State would have a lot more to gain by keeping ND as an annual non-conference game than Purdue ever would.
If recent history is any guide, Michigan State will likely be among the better teams in the Big Ten in the years ahead. And there's a far greater chance that Michigan State will beat ND once every few years than Purdue beating ND. Having the opportunity to beat ND once or twice every 5 years would probably make it worth it for MSU to keep ND on its schedule, as having a quality non-conference win will push MSU higher up in the rankings, sell more tickets to games, increase TV exposure and revenues, and potentially position them for a better bowl game.
The only thing that Purdue stands to gain by playing ND is to have a sold-out game at Ross-Ade every other year. Since Weis became ND's coach in 2005, Purdue has beaten ND only once, and that was during ND's worst season in modern history (2007). Because Purdue has been among the weaker teams in the conference, and I don't see that changing anytime soon, Purdue really needs wins from all of their non-conference games if they hope to become bowl eligible. So they have little to gain by having an almost guaranteed loss against ND each year.