I don't think that this will be a big deal for the kanrethad fight.
Keep in mind when most people were first doing it at the beginning of this tier they were anywhere from 470-490. A lot of people had to pull out crazy tricks to survive.
Fast forward to 5.4 with new gear and item upgrades, among other things...I would imagine the average LFR hero would have at least a 505-510 ilevel going into the patch. While this gateway change will technically make the fight more difficult, the increase in item levels will make the fight significantly easier. Maybe instead of being a skill check it will be more of a gear check. Either way, I think we also need to keep in mind that most of us that wanted green fire have it, or have access to getting it before this change is implemented. Some of us aren't allowed to use green fire in raids. This may come across as elitist, but I don't mean for it to be, however I think its a good thing that something such as green fire is reserved to people who have put a significant amount of time into their warlock.