Some people seem to want a Party which is 100% in line with how they think and feel about politics, and when it does not turn out that way, they leave in a huff. This is an entitled mentality.
In the real world, nothing is perfect. We have to go for the lesser of evils. Singh may not be perfect, but he is the leftmost of the three major alternatives. As is the NDP.
How is the NDP going to reflect your wishes if you leave in a huff? Isn't it better to stay in and fight for what you believe in? If you have any political experience at all on the progressive side of things, you should be used to being defeated. I certainly am. That doesn't stop me from talking to my friends and coworkers about progressive issues...
Jagmeet could be proactive on this He could easily say "the days when the Left is totally out in the cold in this party are done with. Now that I'm leader, activists, for the first time in this party's history, will decide policy and riding associations will have total control over candidate selection". Those two things, plus a housecleaning at party HQ, would do a lot to keep people in the tent, while not driving anyone else away.