The problem here is that the "immigrant" visa is really the same thing as a work visa, and that leads to a great deal of abuse.
For one thing, the eligibility is tied to an employer, who is incentivized to embellish, cheat, and otherwise finagle their way to a visa. The expectation that the visa is temporary also sets a lower bar for entry - even though many of these people will eventually become American PRs/citizens.
The recent trend to "stage" the H-1B green card process is a step in the right direction - though it doesn't go as far as it needs to. There is still a major problem of indentured servitude. Once an employee has a green card process in the pipeline their employer has them over a barrel - and many will not hesitate to use this as an opportunity for abuse.
Not to mention, with the green card backlog the way it is now, it would be years before "your huddled masses of immigrant entrepreneurs yearning to breathe free" are actually able to start businesses. First they have to go through ~8-10 years as a rank and file employee, before they're granted the legal freedom to pursue their own future. The startup visa would go a long way to alleviating that, though the heavy involvement of VCs in that initiative will mean that bootstrappers and other scrappy startups that don't want to raise hojillions in funding will be still at a severe disadvantage.
How do you fix this? IMO the US needs to setup a track for immigrants to immediately receive green cards, where having a job offer is not a prerequisite (though it would certainly help). The focus needs to shift away from fulfilling "temporary shortages" (which we all know is bullshit) to simply permanent, mass importation of worthy talent. Set up the process to filter people based on the assumption that they will permanently stay, as opposed the the current process where we'll let just about anyone in, since they're "temporary" H-1Bs anyways. This will raise the calibre of people you're letting in, and also make sure you're giving top talent maximum freedom once they're here. The whole "immigration policy masquerading as work permit" thing really needs to GTFO.