If you're an assistant, you're focusing on the kick taker and the last man, in that split second the ball is struck you need to consider every player who is beyond the line for offside/onside. It's difficult to judge whether a player is blocking his eyeline (especially when you're seeing it sideways on) as well as taking in the above. Not to mention the defenders you're focusing on, the keeper and the opposition player will be making sudden sidestep movements from their position as the ball is being struck. Personally, I think player reaction is a perfectly reasonable thing to consider in this scenario
You're right that it's difficult to judge LoS blocking from the line - that's why you draw it to the attention of the ref and discuss it. AR identifies the PIOP, ref probably has a better picture of interference, jointly make a decision.
As for 'don't flag offside infringements unless the players call for it' - sigh, let's just make a decision.
The problem with that thinking is that if the keeper appeals for it after a second or two - yes, that WILL happen, you're screwed. Because now you know you've screwed over the team to try to make your pay go a little easier, and you can't change your decision otherwise it will look like you're just disallowing it because the keeper yelled at you.
As I said, how many players are even going to remember that blocking LoS is a thing? How often does that get given at ANY level? The number of times I've seen that be a relevant factor in my entire career I could probably count on one hand.
YOU know the laws.
The players don't.
So make your decision - don't ask the players what the decision should be.
Nothing looks worse than an AR who is clearly raising the flag only because the defending team yelled at him.
Look, yes, there is some scope in considering player reactions for borderline decisions. But if you're on the line, then you don't know how borderline it is. You don't need to stick the flag up - call the ref over, discuss it. Let the ref make the final decision. Referee needs information to make a decision - and that means your information on a potential offside infringement.
If the ref then decides that they don't want to 'influence the game', well, that's on them. But if they reckon there was clear blocking of LoS then the ref needs the chance to make that decision.