Bangladesh can win this game very easily as they have to make just 183 score in the 41 overs, that is very easy for every single team and especially for these Bangladesh players, it is the game of their left hand. They have lost their one wicket but I don't think so it hurt to the Bangladesh team to chase this small target.
Afghanistan didn't play as good as they should have but since, it is the first game of the series and I hope in the further matches, their batting will be back in order as they used to play in World Cups.
It is not right to be sure that Bangladesh will win without seeing the end of Bangladesh's innings. Bangladesh is a team where if there is a batting disaster for some reason, it doesn't take long for wickets to fall. However, Mehdi Hasan Miraj and Tawheed Hriday remain unbeaten on wickets, so for now Bangladesh has a chance of winning. If Bangladesh can bat full fifty overs in a defensive manner then definitely Bangladesh will win. Moreover, Mushfiqur Rahim and Mahmudullah Riyad still have two finishers, so it can be said with confidence that Bangladesh will play well and win for the time being.
You are absolutely right in your concern that we should not assume that Bangladesh have won the match until they actually win it. They need another 100 runs in 17 overs but the problem is that they are losing wickets and now only 3 wickets are left with them. All the set batsmen Soumya Sarkar, Najmul Hossain Shanto and Mehidy Hasan Miraz have lost their wickets and now this task is becoming difficult for them. I think they may lose this match.
Here i would like to mention some super bowling spells by AM Ghazanfar. He took 5 wickets in his 6 overs that he bowled. The famous Rashid Khan also took 2 wickets and the Afghanistan team who were looking that they may lose, are now on the verge of pulling it back and snatching the win from the Bangladesh team.