The Pats have an option on Dillon. Dude is still a load to bring down, even if he has lost a step. The Pats should do something similar with him to what the Rams did with Bruce this offseason. Let him go, but re-sign him from the open market at a cheaper price. I doubt Dillon gets strong interest elsewhere. Maroney gets more work next year, but its not quite time for him to be the true feature back.
Would the Panthers cut Foster? I doubt it. His signing bonus was only 4.5 mil, so it's not a huge hit if they cut him, but he's still a useful player. They have bigger questions to answer than RB, and they haven't shown any inclination to give DeAngelo even half the work when Foster is healthy (despite Foster's middling production and talent). Like Maroney, Williams should get more work than this year, but not a feature back load.
Of course, the big variable here is offseason movement, so this is pure speculation right now. The bottom line is that I wouldn't automatically assume these guys get enough work to be RB1's for fantasy next year. The vets provide stability and security, even if they are unexciting both on the field and on the stat sheet.