Books from the major labels that sell less than 20,000 need to get cancelled plain and simple.
A retitling is not going to save this book. Every book that Marvel puts out that has been retitled has either been cancelled (Dark Wolverine, Incredible Hercules, Black Panther) or on the verge of cancellation (Journey into Mystery, Captain America &). Retitling a book has never worked in the modern era. It's just done to capture the audience that is already buying the book on a concept that would never sell to begin with and then once the reader figures out that the original protagonist is never coming back, they abandon the book en masse.
But what makes this title even more baffling is the fact that unlike Wolverine (65k), Captain America (45k), Daredevil (40k), Incredible Hulk (100k), and Thor (42k), Thunderbolts does not have an audience to build off of. At the rate it's being dropped, it will probably reach anywhere between 17k - 15k by the time it becomes Dark Avengers. In this instance it would have been better to launch Dark Avengers with a new #1 as opposed to taking over a book that no one reads.
Having Jeff Parker as the writer doesn't help matters either. He is forever doomed to be a great writer who will drag books down to a slow, slow, slow death.
I think your wrong
First off some sales figures for Thunderbolts from the last few years...
Month/year
01/06 #17 - 23,417
01/07 #110 - 64,996
01/08 #118 - 43,645
01/09 #128 - 39,219
=====
01/10 #140 - 28,644 ( -4.4%)
02/10 #141 - 33,948 (+18.5%)
03/10 #142 - 34,132 ( +0.5%)
04/10 #143 - 35,656 ( +4.5%)
05/10 #144 - 33,276 ( -6.7%)
06/10 #145 - 31,999 ( -3.8%)
07/10 #146 - 31,501 ( -1.6%)
08/10 #147 - 31,252 ( -0.8%)
09/10 #148 - 32,190 ( +3.0%)
10/10 #149 - 30,580 ( -5.0%)
11/10 #150 - 31,704 ( +3.7%)
12/10 #151 - 27,943 (-11.9%)
01/11 #152 - 26,515 ( -5.1%)
6 mnth (-15.8%)
1 year ( -7.4%)
2 year (-32.4%)
3 year (-39.2%)
4 year (-59.2%)
5 year (+13.2%)
T-bolts has been a lock solid 30k seller for a long time, In fact issue #17 sold only 23k and the franchise was revitalized around #110 with the arcs stemming from Civil War.
The book has proven to show lulls at times and bounce back. Ask yourself with the current arcs in T-bolts "why should people care" ...the roster is bad with Troll, Mr Hyde, Satanna, Boomerang, Etc.. It's not going to win people over. The book has an irrelevant feel to it.
Now ask yourself, what do we get if a book like this is cancelled...is marvel's formula working?
I'd argue no...the double ships and spin offs from "money books" certainly is no lock to work....
Some sales figures for around the 2010 period of some of these type books..
IRON MAN LEGACY
04/10 #1 - 48,956
05/10 #2 - 29,175 (-40.4%)
06/10 #3 - 23,340 (-20.0%)
07/10 #4 - 20,142 (-13.7%)
08/10 #5 - 17,773 (-11.8%)
09/10 #6 - 15,949 (-10.3%)
10/10 #7 - 14,882 ( -6.7%)
11/10 #8 - 13,686 ( -8.0%)
12/10 #9 - 12,483 ( -8.8%)
01/11 #10 - 12,349 ( -1.1%)
6 mnth (-38.7%)
Cancelled with issue #11.
THOR: FIRST THUNDER
09/10 #1 of 5 - 21,228
10/10 #2 of 5 - 16,126 (-24.0%)
11/10 #3 of 5 - 14,038 (-12.9%)
12/10 #4 of 5 - 12,620 (-10.1%)
01/11 #5 of 5 - 11,799 ( -6.5%)
More movie fodder.
X-MEN FOREVER
01/10 #15 - 17,665 ( -3.9%)
01/10 #16 - 17,290 ( -2.1%)
02/10 #17 - 17,873 ( +3.4%)
02/10 #18 - 16,464 ( -7.9%)
03/10 #19 - 16,183 ( -1.7%)
03/10 #20 - 15,990 ( -1.2%)
04/10 #21 - 15,530 ( -2.9%)
04/10 #22 - 15,305 ( -1.4%)
05/10 #23 - 15,045 ( -1.7%)
05/10 #24 - 14,940 ( -0.7%)
06/10 GS#1 - 15,039 ( +0.7%)
06/10 #1 - 16,094 ( +7.0%)
06/10 #2 - 14,933 ( -7.2%)
07/10 #3 - 14,151 ( -5.2%)
07/10 #4 - 13,835 ( -2.2%)
08/10 #5 - 13,632 ( -1.5%)
08/10 #6 - 13,351 ( -2.1%)
09/10 #7 - 12,869 ( -3.6%)
09/10 #8 - 12,753 ( -0.9%)
10/10 #9 - 12,260 ( -3.9%)
10/10 #10 - 12,064 (-16.0%)
11/10 #11 - 12,018 ( -0.4%)
11/10 #12 - 11,756 ( -2.2%)
12/10 #13 - 11,419 ( -2.9%)
12/10 #14 - 11,192 ( -2.0%)
01/11 #15 - 11,101 ( -0.8%)
01/11 #16 - 10,937 ( -1.5%)
6 mnth (-20.9%)
1 year (-38.1%)
X-MEN: TO SERVE AND PROTECT
09/10 Vampires #1 - 25,654
10/10 #2 of 2 - 23,362 ( -8.9%)
11/10 Serve #1 of 4 - 19,273 (-17.5%)
12/10 #2 of 4 - 15,976 (-17.1%)
01/11 #3 of 4 - 14,530 ( -9.1%)
RON MAN/THOR
11/10 #1 of 4 - 21,416
12/10 #2 of 4 - 15,977 (-25.4%)
01/11 #3 of 4 - 14,139 (-11.5%)
CAPTAIN AMERICA: MAN OUT OF TIME
11/10 #1 of 5 - 23,566
12/10 #2 of 5 - 14,582 (-38.1%)
01/11 #3 of 5 - 13,060 (-10.4%)
Movie fodder.
I AM AN AVENGER
09/10 #1 of 5 - 20,922
10/10 #2 of 5 - 14,882 (-28.9%)
11/10 #3 of 5 - 12,277 (-17.5%)
12/10 #4 of 5 - 11,242 ( -8.4%)
01/11 #5 of 5 - 10,520 ( -6.4%)
THOR: THE MIGHTY AVENGER
07/10 #1 - 20,076
07/10 #2 - 14,315 (-28.7%)
08/10 #3 - 12,112 (-15.4%)
09/10 #4 - 10,887 (-10.1%)
10/10 #5 - 9,673 (-11.2%)
11/10 #6 - 8,420 (-13.0%)
12/10 #7 - 8,244 ( -2.1%)
01/11 #8 - 8,323 ( +1.0%)
6 mnth (-41.9%)
I could put up more of this stuff...as it goes on and on.