What makes you think that the present system is not exactly what you have described?
It is what I've described:
Desperately poor people risk their lives to enter illegally, then live in the shadows in constant fear of deportation, if they survive the trip.
That describes what is happening.
The reason that it is happening is that the federal government is not doing the job it is supposed to do, namely, securing the border and monitoring immigration.
The chances that a Mexican Campesino is going to get to immigrate legally is somewhere between zilch and zero. Meanwhile, farmers here depend on the illegals to harvest crops before they rot. There needs to be a guest worker program in place and working in which the poor of Mexico can fill those seasonal jobs, get paid a fair wage, not have to dodge the Border Patrol, and then return home with their earnings. There needs to be a way to limit the number of people crossing the border, and a way to make sure that the drug dealers and gang organizers get left behind. As it is, there is no provision for any of that.