Avoiding thunderstorms is a good idea no matter what kind of plane you're flying... That's just common sense. In the meantime, it should be noted that the 787 has survive multiple cases of direct lightning strikes with no major incidents resulting, and even more important, the planes struck by lightning have been quickly inspected, repaired, and returned to service.
As for ice accretion inside engines, this isn't just a thing with 787 engines. All high bypass motors and many low bypass motors are susceptible to various forms of engine icing, whether it is inlet icing or blade ice buildup. I strongly suspect that in this case, they'll do a little more testing, provide some temporary guidance on how to avoid the conditions that cause more rapid than usual buildup, and when a permanent fix is developed the engine manufacturer will make the fix available for retrofit as the engines go through routine servicing. There's nothing unusual about that process... I don't think there has ever been an airplane ever designed and built that hasn't had something like this go on. Something pops up in real world ops that wasn't seen in testing, they re-test to determine the scope of the problem (if there is one), then turn the engineers loose to fix it.
Being told to avoid thunderstorms... That's a bit like being told to not wreck your car parking it in the garage every day after work. It's something everyone does anyhow, and if you bump the garage wall once or twice and bend your front license plate, you don't throw the car (or the garage) away... You figure out a way to quit doing that or ignore the problem and accept the occasional bent license plate and wall scuff mark.
