Yes, realistically the NDP is competing for 2nd place not 1st.  

If Scheer and the Conservatives win in 2019, it will probably mean that many youngsters blinded by the Trudeau 2.0 charisma in 2015 (as I was by the Trudeau 1.0 charisma in 1968) have returned to the NDP fold or didn't bother to vote.  For Singh to finish 2nd, he must get these disaffected voters to get out and vote NDP.  Among the NDP leadership candidates, he would seem to have the best shot at doing this.

If the Liberals win in 2019,  The Dauphin's base among young left leaning voters somehow held, the Conservatives will finish 2nd and the NDP 3rd again.

We need to know more about Singh's platform and capacity to debate, inspire and lead but he seems on the surface to have the best change to get his party back to 2nd in 2019, positioned to go for gold in 2023.

I don't support the NDP but why do you claim Singh is not a leader?  Compared to Trudeau, my border collie is a leader.