High immigration rates could have saved Detroit. 

As for crime,it requires two ingredients - a large percentage of unemployed young males, preferably born to single mothers. Also, crime requires a lot of income inequality.

If Toronto gets both in 30 years, high crime rates are possible.

Finally, Toronto's crime rates are a lot lower today, after 40 years of high immigration rates, than they were before. Winnipeg, Regina and other low immigrant percentage cities suffer much higher crime rates than Toronto, for the reasons listed above.