The problem there is that the phrase "line of sight" is often used to also mean "line of effect", such as the Fear example; and that gets confusing when windows are involved. I have line of sight to you through a window, as in I can see you out there, but not line of effect as there's a physical obstruction in the way.
Consider the following sequence (let's assume an open field with no cover, for simplicity):
1. I hit you with a Fear effect, you blow your save and start running away
2. Someone else hits me with a Blindness effect
3. Because I'm now blind, does that mean (or more importantly, should that mean) you're no longer scared of me even though you can still see big scary old me over there? IMO no it shouldn't.
This could be solved if it were noted that line of sight works in both directions; if you can see it or if it can or could* see you, there's line of sight.
* - if it had eyes, to cover situations when the target is an object
It's not the
caster being blinded that alters the
fear spell. It's the
target being blinded.
If you cannot see the thing you're afraid of, you don't know which direction to run. That seems perfectly adequate for a spell
of the Illusion school that causes someone to be so frightened they run away.
If you can't see the illusion that you're supposed to be frightened of, why would you still be frightened? Obviously being blinded would be a real pain and not fun, but it wouldn't be "brain has stopped working, please leave a message after the AHHHH!!!" fear.
Edit: And yes, if the spell writers (or editors) have been so sloppy as to conflate line-of-sight with line-of-effect, then of course there are going to be problems. It is possible to see something you cannot affect. It is also possible to affect something you cannot see (e.g.
sickening radiance explicitly says that it "spreads around corners", so it clearly can affect targets you can't see that are physically behind a solid wall.)
Yet another place where treating "natural language" as though it can bear the burden of specificity and clarity leads to a serious problem, if they were so foolish as to conflate these things. Even I, as critical as I am of 5e, would need to see an actual citation for that.
Edit II: Electric Boogaloo
Having checked, 5e does not
explicitly use the phrase "line of effect"....but in the Spellcasting rules it lays out everything needed for the definition thereof. That is, every spell has a point or object/entity of origin, with a specific shape of area of effect, usually cone, cube, cylinder, line, or sphere starting from that point. Within the defined area of effect, the spell's effects extend along
unblocked lines (really "rays" but I won't quibble too hard) from the point of origin. For something to block a spell's effects, it must provide total cover--so things which do not provide total cover do not block a spell's effects. It doesn't explicitly say so, but the existence of spells like
sickening radiance implies that this unnamed-but-defined line of effect rule allows exceptions with spells that specify that they spread around corners.
Functionally, this means 5e has the rules for line of effect, without ever using the
phrase "line of effect".