Even offering it to illegals who arrive before the program is in place will do nothing to slow down illegal entry.
And holding out the possibility to those who are contemplating illegal entry, that they might be able to participate in the program if they make it into the country before the program goes into effect, will encourage them to try to enter illegally.
This of course is completely separate from the question of whether to offer it to aliens who haven't yet entered the country, as a way of allowing them to enter legally. That's not what I'm commenting on here.
Of course. But there are ways to discourage that. Again, employers are the key. If the plan says that illegal immigrants can't prove that they have worked for the same employer for a year or two, they won't be able to get a job, and will be deported, then it won't encourage many to try to get here except through the legal program.