You're assuming that because G-YMMM was affected it must have been particularly prone to such a failure(whatever it was).
From a statistical point of view this is not a valid assumption and the incident might actually be equally likely on other aircraft types.
We're talking tiny probabilities here and a single incident is not statistically significant.
So without some idea of the cause it would be difficult to justify taking action against a single aircraft type.
Well said.
You could also add that if you restrict/ground a particular aircraft type over one incident and subsequently are unable to find anything in particular that is wrong with it, how do you logically go about de-restricting/un-grounding it? Nothing has changed in the interim yet you'll have to make a 180deg. turn in terms of decision making whilst in possession of the same facts...
This also impinges on why there are no significant procedural changes in the operation of the aircraft. If you don't know what caused the problem, how can you know if your 'solution' is making things better or worse? Airlines and manufacturers are reluctant to provide 'instant fixes' as they often come back to bite them through the law of unintended consequences.
At the moment we don't know if what befell MMM was because of:
a) something unique to that airframe
b) something that could happen to RR-powered 777s
c) something that could happen to all 777s
d) something that could happen to Boeing aircraft
e) something that could happen to all commercial jets
f) etc...