I think you’re spot on with your analysis, although I think a good argument can be made for simply keeping him for the rest of this year as opposed to cutting bait, now. May as well have a capable backup should injury arise. Like you said, none of us want to see a repeat of the Steven Jackson situation.
NOW
There is absolutely ZERO reason to cut Gillislie now. We need the backup in case of an injury, or even two.
I don't see the problem. We brought in 2 backs: Burkhead and Gillislie. Lewis game back strong. One of the two additions is inactive.
THE PHILOSOPHY
I don't understand. We have four capable RB's. There may be game plans where they are all active, but I would expect that one would usually be inactive.
Early in the year, Lewis was playing relatively few downs, as his body was being tested. Eventually, he became our #1 back, beating out Burkhead and Gillislie. White is a fine 3rd down back. Burkhead (who we pay lots to) was seen to be a better to be active than Gillislie.
What SHOULD we have done? Should we have counted on Lewis, some new guy and White, and hoped for no injuries?
There is NO problem with one of our backs being inactive. It is a GOOD think to have a solid backup, especially at RB.
2018
Will Gillislie be worth $2M for 2018? IMHO, it would be silly to decide this before being very deep into camp.
I should note that Lewis is not signed for 2018. Also, Burkhead isn't signed for 2018.
IMHO, it would be great to re-sign Lewis and Burkhead and have the same RB's in 2018 as we have this year.