Modern countries have modern penal and justice systems. They evolved for the population in that country and its needs.
If you bring millions of criminal migrants into your country, a number of things can be expected to happen:
- The jails will start filling to maximum capacity
- The prison system will put pressure on the courts to not impose custodial sentences
- The deterrent of jail is reduced
- More and more commit crime, clogging the courts
- The courts put pressure on the police to administer warnings instead of prosecutions
- Law enforcement is reduced
- Even more crime of an even more serious nature is committed
There will soon be no effective prison or judicial system in the country, as it will be entirely overwhelmed by the amount of cases.
To see this in action, look at Europe.