I know that the NFL faces a difficult legal situation to offer live games on the internet but they already do that in Europe and other parts of the world.
I wouldn't characterize it as a "legal situation" in the US so much as a case of having contracts with the networks which prevent it. The networks' US rights to exclusivity are broad and ironclad. This Yahoo broadcast is possible because either (1) it falls in some narrow margin outside the contract exclusivity, perhaps by virtue of being broadcast in the teams' local markets and that it starts at 9:30 AM Eastern time or (2) perhaps the NFL bought back or swapped something for the "national" rights from a network in order to sell them to Yahoo.
In any event, as a network contract for US rights expires, there's no contractual or legal issue I can think of preventing the NFL from producing and delivering themselves their content exclusively over the internet.
The issues are:
(1) Market penetration of high speed internet and wifi-TVs as opposed to nearly 100% market penetration today with at least a tube TV and rabbit ears, the primary exceptions being the tiny minority who don't own a TV as a lifestyle choice. There will be a tipping point. In my market, the cable provider offers high-speed internet for $15 per month with a guarantee the fee will not be increased (in other words, it's not a teaser rate). A person of modest means could dump his conventional cell phone contract, switch to a pre-pay, and buy a high speed service at this price and save money in the bargain. Also, some people of modest means have not upgraded from their tube TV. In the not distant future, only a tiny minority of TVs in operation will lack wifi capability (or at least a USB port where you can plug in a wifi dongle for next to nothing) as tube TVs and early generation flat screens die off.
(2) After that, the biggest hurdle is political.
Living in a market where the team has not made the playoffs in 15 years and where the weather can be brutal during late season games, threatened blackouts have been common. While local businessmen usually step up and buy some thousands of tickets or the team will buy them themselves, it's a dirty little secret that the NFL allows teams to sharply discount the tickets in bulk sales at a certain point before game time or the teams can buy their own tickets for a fraction of face value. Also, visiting teams split the gate something like 60/40 excluding some premier seating...not filling seats at full price affects every team.
The NFL's line on suspending blackouts is that the TV rites revenue is now big enough to offset the loss of ticket sales in blackout scenarios. Well, that's been true for awhile. This is largely a political issue. There is a strong sentiment in Buffalo (and I imagine the 5 or 10 other markets that struggle to sell out at one time or another) that taxpayer support for NFL stadiums grant the populace the "right" to free viewership of every Bills game. That's what I meant by it being nearly a Constitution amendment that everybody has the right see games on TV for free.
Whenever a threatened blackout rears it's ugly head, it's a big issue in this town, the writers write about it, and fan ire turns toward the team that took public money and the politicians who support it.
Further, the NFL (along with the other US professional leagues) operate under anti-trust exemptions granted by Congress allowing, among other things, player drafts and salary caps. Whenever Congressmen get heat over some issue in pro sports (e.g., drugs, concussions), they conduct a show trial, dragging the sport into session for interrogations. The sports inevitably take some action, even if modestly and slowly.
In other words, pro sports in the US operate at the pleasure of Congress with anti-trust exemptions held over their head as a cudgel if our erstwhile representatives start to feel political heat over some issue.
If the NFL went exclusively to an internet model because they could simply make more money that way, while cutting out a meaningful percent of the population who have not embraced the necessary technology (just as they cut out the viewing public in past blackouts) resulting in a sufficiently loud hue and cry, there's a good chance Congress would find some pretense to come down on the league with a hammer.