One page to solve many problems
I like Greta's simple approach a lot better than the 1200 page Gang of Eight nightmare, but the general topic of "immigration reform" actually covers several issues, which is why efforts to solve the problem tend to grow very complicated.
For example, what about illegals who have committed crimes besides crossing the border illegally, such as identity theft, falsifying documents, and drunk driving? Do they get automatic green cards, too? If not, it will be necessary to set some standards, and that would become another bitter political battle, not to mention greatly increasing the complexity of the simple immigration solution. (Hint: get ready for a huge battle over excluding drunk drivers from the green card giveaway. It's not a coincidence that the Gang of Eight bill indulges them to a ridiculous degree.)
I presume part of Greta's solution would involve cracking down on anyone who doesn't have a green card, along with employers who hire those without proper documentation. That, too, would turn into a sizable pile of legislation, and it would be necessary to avoid all the "Administration discretion" loopholes peppered through the G8 bill. The federal government, particularly this Administration, really does not want to enforce immigration law or deport anybody, so it cannot be trusted with the discretion to waive any portion of a comprehensive reform. Look at the reaction when state governments talk about serious efforts to detect illegals.
If strict enforcement of green card documentation is part of the border security package, then eventually you're going to see photos and video of an undocumented family deported, and the whole "comprehensive immigration reform" argument would begin anew, portrayed as the great civil-rights battle of the era. It will essential to describe exactly how the undocumented will be detected and dealt with, and that's inevitably going to make Greta's straightforward proposal more complex.
The green card idea isn't going to satisfy those who want full-boat citizenship for the young "dreamers" and their families. (I've always been sympathetic to the people who were born on American soil to illegal immigrants, because they did not make a conscious decision to violate our immigration laws, but it's tough to conjure a "pathway to citizenship" for them without inevitably including their families.)
And the green card proposal won't satisfy those who want to give illegals access to social services, or of course those who want them as voters, so it's a dead letter anyway. That's not Greta's fault, but political reality is a stubborn thing.