Personally, I don't see a big reason to hold onto him, unless he's willing to take either short money (1-2 year deal) or a 3 year deal at about what we were paying him last year.
At the very least, he doesn't want to do either. Time will tell if he has better options.
Personally, I think Sullinger and/or Johnson can do a lot of what Bass can do. While I appreciate that Bass performed his job so well, let's remember that his job was essentially to hit wide open jumpers and wide open dunks/layups. Being a 5th option with Rondo and 3 Hall of Famers has its perks.
Now you can argue that Bass filled this role better than anyone in the Big Three Era (save perhaps Shaq); however, let's also not pretend that Bass is any sort of reliable scorer that could be depended on to create his own offense or carry the team.
Thus, I think that if the C's were willing to bite the bullet early and deal with some early growing pains, Johnson and/or Sullinger could be worthy replacements and potentially even better.
Johnson is a good jump shooter out to 15-18 feet. He also finishes around the basket well. Thus, he could potentially perform two of Bass's roles well. And unlike Bass, Johnson is more of a shot blocking threat.
Sullinger is also a good jump shooter. And while perhaps not a athletic as Bass, he possesses a wider array of post moves. He potentially could be an option on offense to create his own shot.
So again, I wouldn't totally dismiss what Bass gave us. What he did he did very well. However, I don't think he's irreplaceable. And if he's not irreplaceable, we shouldn't pay him like he is. I'd love to see him back, but not at any sort of salary that would jeopardize our future flexibility.