It's not that huge. Remember how many attempts it took for Paragon to down Ragnaros HC, about 500. So even if Ra-Den is easier and you'll "only" need 300 attempts, that means 10 Weeks, or about 2 month, where real cutting-edge guilds do 500 attempts in a single week. Thus, it will lengthen then content by far. And it's false to think most guilds extended lockouts. If you have got so far into progression, that you can attempt on Ra-Den, you will not waste much time on defeating the previous bosses. Once killed, these guild have those bosses on farm and to farm 13 bosses in a single raid instance will not take so much time. Farming T14 nowadays takes about 2 evenings with a normal schedule (meaning about 4 hours a day). So even if you need let's say 20 hours to down all bosses on you way to Ra-Den cutting-edge guilds will take their time to do it in two days, so they have five more days for only 30 attempts.
The whole attempt-model is just dumb, especially with a not tested encounter like Ra-Den. I remember For the Horde facing Sinestra for the first time (as the first guild) and wasted many attempts, just to see, who bugged (since not tested) this encounter was. If you have only 30 attempts, would you like to see, that you first need to waste 10 or 15 just to work out what the boss do and then to see, it will be altered on the fly, because of the bugs, noone knew about? Sounds funny.
The only thing, this will do, is to make the world-first-race closer, as guilds not WILLING to invest as much as the current top-guilds can catch up and will only be a few attempts behind. And although I'm a member of one of those guild and I'm really not WILLING(!) to invest so much time (and also just can't), I think, more investment should yield more profit. But so the cutting-edge guilds invest much more time than anyone else for the first bosses, and are than stopped by a artificial barrier called "limited attempts", so players like me, who invested so much less time and effort, have a much more realistic chance to kill this boss first (okay, not me personally, but top 10-20 guilds).