If you are a Marvel fan you know that there are sequels planned. That almost goes without saying. They've announced them. If you've read the comics then you'll know that "Infinity War I " tracks the basic storyline, but there are changes.
Nevertheless Marvel is so successful at casting the right actor for each character, that the fans could not but help empathize with the loss of that persons life however fictional it is on the screen. You suspend your "disbelief". Which is what all very good stories manage to do. There is a measureable sense of loss. I know I felt it ,even when I have a reasonable sense of what is to come in the next movie.
There is no doubt however that some of these deaths are permanent, others not so much. Some of the reactions have been very charged, particularly about the Black Panther ,I've read some ridiculous comments, BUT that only goes to prove how hugely popular he is/was/has become in a very short space of time. That is the power of a good movie it elevates the character and the story. Can you imagine the strength of feeling there is going to be within the next film because of all that has been lost in this one, when those heroes are returned to us?
However , for certain characters to return ,there are very likely going to be further losses very likely from those surviving Avengers and they will hurt. If this one has kicked us in the guts, I suspect the next is going to both heal and break our hearts simultaneously, which is quite the trick when you think about it.