The USFL had established itself in the spring during its three seasons, and probably could have continued indefinitely with that schedule, if its owners had not inexplicably decided to follow the lead of a certain orange-hued monster and attempt a move to the fall. Even if the antitrust trial's jury had been properly instructed and had awarded the league an appropriate amount of damages after the court victory, the league's sailing would have been rocky in the fall, as the move itself cost the league its teams in Philadelphia, Houston, and New Orleans.
The World League / NFL Europe lasted for about fifteen years. This league was run by the NFL itself; it produced one star player (Kurt Warner, who actually got his start in the Arena League) and several other players who had good NFL careers. More important, the league's original World League incarnation showed that the logistical hurdles of a league having teams in both North America and Europe are surmountable, a point which is often forgotten by those who discuss the possibility of an NFL team in London.
Neither of these leagues had experimental rules during ordinary play, though the NFL tried out what would become its new overtime rules in the World League before adopting them itself.
From a purely branding standpoint, the NFL probably should have kept NFL Europe going, as it brought American football with the NFL name in person to new audiences on a regular basis. I suppose the NFL figured that anyone in Europe who cares about the NFL can be served by international broadcasts of its regular games, and that London was sufficient for the European staging of matches.
American fans have not had a stable spring outdoor league simply because no television network has been willing to buy broadcast rights of an independent league since ABC paid to show USFL games. By contrast, broadcasts of the UFL were time-buys which not only generated no revenue for the league but burned through the league's meagre coffers.
We shouldn't overlook the Arena Football League, which has been on television and in American cities in the spring for thirty years. The league in its early days usually was able to get some scant money for its television rights, and then in the early 2000s entered into a high-profile revenue-sharing partnership with NBC. However, this did not succeed in establishing the league in the sports mainstream, and the league folded in 2008, a few years after the end of the NBC partnership. A new (and admittedly inferior) version of the Arena League began in 2010, and has paid for its television coverage since then.