Why didn`t the producers fire Robert Beltran?
1. I think they noticed that there was a very big fanbase of fans who liked/adored him (Chakotay). When I became a fan of Voyager in 2005 (only because I recognised RB in Caretaker) I searched the net about him. And 5 years after Voyager`s ending there still were many, many fansites about Robert Beltran - various RB shrines; Robert Beltran Estrogen Brigade; some Inofficiell Fansites; RAB; ...- I don`t know all of the names. Most of the sites went down over the last years.
2. The relationsship between Janeway and Chakotay was /is very importend for many fans. And I think the producers knowed it. I know, many fans would have been very upset about Voyager without the tension between J and C.
I can hear (not necessarely agree with) Robert Beltran's frustration concerning the treatment of his Chakotay's character but I could too say the same about Kate Mulgrew and/or Ethan Philipps, whose the characters were damn & seriously damaged through the years too : Janeway was often a dismissed or belittled as a woman in her leadership role with ridiculous moments like short-sighted decision making and poor choices... even if all Captains had their moment ( from TOS, TNG, DS9 to Entreprise! As for Neelix, the poor guy was considered a pedophile because of his propension to spend a lot time with VERY young people, firstly with Kes during 3 seasons then with Naomie Wildman (and all others children on board) during 4 seasons and his excess of smile and good humor!

However, we never heard/read any public critics from them during the 7 seasons of the show, because they were well aware of the chance they had to be there, on a long run show and generously paid.
-> The only time where Mulgrew turned to the press, was at the end of S5, when she declared wanting to spend more time with her family even if it meant to leave the show, when her repeated requests to rearrange her schedule failed ... and it worked because well, I guess that UPN/producers perfectly knew that 1) her decision to quit was not a threat in the air and 2) she has always cooperated with everyone from the beginning to all of her business responsibilities/obligations on & outside sets.
-> As for Philipps, his leaving at the beginning of S7 was not a big deal because it was led intelligently: Philips thought having been around his character - opinion shared by producers, but Neelix was quite popular too so, I'm sure UPN's executives and producers would have found a way to keep the actor until the end. Plus I don't doubt that the actor was maybe a little frustrated to see that there would be no chance of improvement in his character but well, his departure from the show has gone however quite smoothly so, if Beltran wanted to leave, it wasa feasible solution. Some others ST actors did before without drama.
Now, tell me what Robert Beltran's unprofessionnal attitude (critics in the medias & behaviour on/out) brought him?! Nothing of what he expected/wished for that matter, right?! = more money for sure but no noticeable change in the treatment of his character, no say in Endgame's script and I'd add that he was forced to beg the production for something, whatever, to do in the finale => Result: it was given to him, a ridiculous storyline like a nonsensical romance with the culptural blonde beauty at the end, what is rather light if you want my opinion for a lead actor who is supposed to play the brave, wise, experienced and loyal First Officer on a vessel which seeks to make its way in an area infested by Borg Cubes!

And the worst in this story is that Beltran trained with him, Jeri Ryan, who didn't requested anything, damaging in passing, the positive evolution & strength of Seven's character, which reflected since her arrival on board to just become the tall and luscious blonde with whose Chakotay will leave Voyager! How sad!
