At the price? Going into last year when Bus volunteered to take a lesser role he also volunteered to reduce is check to slightly over league minimum, admittedly league minimum for a 10+ year vet is pretty tasty but it's less than 1/53 of the cap so it shouldn't be too hard to afford unless the team is in cap trouble from deferred salary.
Thomas and Allen were both supposed to be feature backs, if your feature back isn't putting up big yards that's a problem, Bus isn't a feature back anymore.
Allen was a back the Bears turned to when Ennis busted out. He was just a substitute from whom little was expected but turned out to be a pleasant surprise. Thomas was rookie of the year, led the Bears in rushing his first three years and averaged about 900 yards a year. This was not good enough and they let him go.