It might not change the overall ecosystem that much.
Many companies doing ICOs are already locating themselves in friendlier jurisdictions, and making some effort to block U.S. contributors.
What efforts are these? All I've seen are checkboxes on their web site making the user tick a box confirming that they aren't a US citizen.
Geo-blocking US IP addresses won't work either, because an ethereum contract can't refuse a transaction based upon its source IP address (it doesn't know it). So if an ICO publishes their ethereum contract address online (e.g. via a twitter post), they are implicitly advertising to US investors.
The most serious effort I've seen had a geoblocked website that worked with a Chrome plugin. You could get around it, without even using a proxy, but only if you could understand the smart contract code and figure out how to prepare the data it required.
To my way of thinking, if you're sophisticated enough to do that, you're sophisticated enough in blockchain matters to make your own investment decisions.
It's a common fallacy that people think "I'm good at X, so I must also be good at Y" It seems to afflict techies the most, somehow we think just because we can understand computers, we must therefore be skilled in other domains too, like investments...
I'm not claiming that being good at computers means I must be good at investing.
I'm claiming that if I understand smart contract code, I'm good at evaluating a white paper's technical claims about a project being built with smart contract code.
Also I'm claiming this makes me better prepared to invest in these projects than, say, a cardiologist. But the SEC would be perfectly happy to let the cardiologist invest, while restricting the programmer who actually understands the industry.
They're already doing it. It's near impossible to do it effectively, if the contributor is comfortable interacting directly with smart contracts, but they're doing things like geoblocking their websites, making you click a box saying you're not in the U.S., etc.
Geo-blocking US IP addresses won't work either, because an ethereum contract can't refuse a transaction based upon its source IP address (it doesn't know it). So if an ICO publishes their ethereum contract address online (e.g. via a twitter post), they are implicitly advertising to US investors.