Might removing sentries have the effect of teams being more reluctant to push out of last? The ability to have a scout swap to engineer and throw up a gun acts as insurance against a failed push out. Of course, this isn't something one thinks about when pushing out, but as teams get punished for pushing out a bit more, it seems to me that over time they will be slightly more reluctant to make the attempt. Might this not also have the effect of prolonging last stalemates?
I don't claim any expert knowledge; this is merely a hypothesis for you more experienced fellows to examine, then affirm or refute.