Honestly, fuck that noise. Play with the 5 invited or don't play at all. It's harsh, but so is life. Gotta keep it fair for every team participating. Last year, LGD (iirc) had a roster change late and lost their automatic invitation. They had to play through the qualifiers to get to TI3. Now we have Fnatic, who is essentially saying "we'd really like to be able to play with whichever of these 2 guys is fit to play" only a couple of months away (now 1 month away) and putting Valve in the position to how force them to at least have to earn their way in. At best, Valve puts them in the final qualifier group with CIS, Liquid and whoever...but that's not fair to those teams who actually won their regional qualifiers.
Gotta have standards and expectations, and you have to hold everyone to them. How much more so with millions of dollars on the line?
Era plays or Fnatic doesn't. To me, it's that simple. Valve would be doing them a favor (and something unfair relative to every other potential participating team) by letting them keep their invitation and play with a stand-in. If your tennis duo makes it to the Wimbeldon Doubles Finals and one of the two goes down with an arm injury before the final match, you don't get to pick some random person to play. You forfeit. Cold rules are cold, but fair.
All that being said, I do hope that whatever Era is struggling with is something he can recover from. Missing TI4 may do him more harm than good, depending on his metal state. I hope the doctors advising him are fully competent and he's getting good care. Being from a medical family, I've met enough psychologists and psychiatrists to know that there are a shockingly high number of shitty ones. And sometimes it's extremely hard to measure success and improvement.