I can see Goins filling the same role he did on the 2015-2016 team, where he was about the 13th man yet still produce in a limited capacity when called upon (like 15 rebounds in the game vs Louisville) Tum will still need to play some as the backcourt depth won't be great next year, unless he sign Smith or McQuaid gets a role again as a secondary ball handlers. Maybe he can make a step up similar to Travis Walton in his senior year as far as filling a leadership/defensive role.
As for the decision itself, great for MSU obviously. I give him a lot of credit for ignoring what most others were probably telling him and making the decision that he wanted to make. I just hope he doesn't end up regretting it and it having an adverse effect on his draft position next year. With the way the NBA folks like to nitpick all your flaws the longer you stay in school, it could happen.