I think you had a typo "we've NEVER closed the game out with the bat before".....is what i think you meant to say.
If you remember the India match in the last world cup....we closed it out with the bat, right?
Apologies if that's not what you were referring to. IMO we've been very very lucky to have played lower-ranked teams like NZ and Zim that simply could not cope with our conditions. Coupled with our now world-class ODI bowling attack and Tiger-like preadatory instinct we have been able to eek out victories against these sides.
IMO against the better ranked sides, unless you get early wickets while defeding, its hard for Bangladesh to win. We will not get rid of the habit of losing wickets in bundles anytime soon, so its better to get the opposition out for as low as possible and let the batsmen play according to a more directed plan. Our batting requries minimum risk-taking. Setting a target ALWAYS involves risk-taking.