I’m really tired right now, and should probably let this ask sit, but if I do the knowledge of it just resting there in my askbox, waiting, will nag me.

There are a few big implications.

1. It prevents, limits, or slows attempting to subvert democracy by sabotaging (or ““sabotaging,”” depending on your morality) the immigration mechanism.  “Just bring in millions of people who will vote for my party” is a potentially exploitable flaw in conventional democracy which could, potentially, result in an unstoppable feedback cycle.
   
1. In this vein, it dis-incentivizes political parties from deliberately bringing in “undocumented” immigrants because they have far less to gain from it.
2. It also disincentivizes the immigrants themselves from coming somewhat relative to a pathway citizenship, although I suspect most are chasing money rather than political voice.
2. It risks creating a long-term, permanently-disenfranchised underclass who cannot vote in their own interests.  This depends significantly on the implementation.
3. Relative to current policy, it accomplishes some of the same goals as a pathway to citizenship, but changes the voting demographics more slowly (one generation vs more-or-less immediately).  So if you were hoping to reduce criminal activity and exploitation through naturalization, you could do most of that with permanent residency.

Of course, a major part of why there is illegal immigration into the country is because you can exploit them in ways that you can’t exploit the natives - not just because the natives can vote, but because people who violated immigration law have to stay under the radar to avoid potential deportation/etc, and so will have to be much more desperate before turning to law enforcement.

The businesses supporting this out of a desire for cheap and exploitable labor do not actually want these people properly naturalized, because then they’d have to pay them more, so they’d tend to just import more unauthorized labor.

It’s important to remember that a pathway to citizenship does nothing to stem the flow that caused this in the first place, and is thus not a real solution.  

Immigration cannot solve global poverty while the rate of new people created each year massively exceeds the capacity of developed nations to take them in.

Only developing the countries of origin economically, such that they are wealthy enough that people mostly don’t want to leave, can truly solve global poverty.

There are also other policy alternatives.

• Issuing a large number of long-term work permits (8-20 years) based on the number of unauthorized migrants currently in the country.  This would allow some of the same effects of bringing people up to the surface, but without the same commitment as permanent residency or citizenship.  However, it is sorta kicking the can down the road.
  
• Allow the individual states to issue a number of permanent residency permits proportional to their population, at their discretion.  I have joked about this, but it’s actually an option worthy of consideration.
• Implement hourly wage-subsidies for low-wage American workers, but only for citizens.  While this is useful for the poor, and may significantly increase their negotiating leverage, it may even have some business backers.  Driving down the de facto wages for all non-citizens, who are not eligible for the subsidy, makes coming and staying far less profitable (and thus desirable) for unskilled labor.
• Allow more immigration above current regular levels, but all of these additional immigrants must be sponsored and insured by specific American citizens/charitable organizations, including any education they may require to meet basic levels.  (aka “if you want this so badly, you pay for it”)