I personally don't think the average guest will care too much. By "average guest" I'm referring to those who are making their first trip, or those who only go every now and then but aren't big die hard fans like us. My signature shows the trips I've made (with 2003 being the first time I went with a spouse), and we didn't see Fantasmic until 2007. We didn't really care to see it the first couple times, then decided to do it in 2007 and enjoyed it.
I think your average guest falls into the category of just going to WDW, have fun, see what's there, etc. IMO, most of them won't care about Fantasmic not being available, especially if it's their first trip, because they won't have any experience seeing it, and therefore, won't miss it. It would be like my wife going for the first time in 2003 and finding out that Skyway is gone. What difference does it make to her? She's never been there before, and therefore has no attachment to it, and therefore is only concerned what she sees in front of her right now.
And for many who HAVE gone once or twice, they're likely to chalk it up to things changing. When I went back to the MK in 2000, after 13 years, I noticed that the Skyway was gone, but it wasn't that big of a deal. And in subsequent trips, other things have changed. I wasn't thrilled with the replacement of El Rio with Grand Fiesta. I wasn't thrilled with the refurb of the Living Seas, or the audio replacement of the tour guides in Living With the Land. But I sucked it up and accepted the fact that things change and that WDW is never going to be exactly as it was when my wife and I had a blast together on our honeymoon in 2003. On our first couple trips, there may have even been some things on the programs guide that were unavailable that day and we missed them until the next trip. But we still had fun with what was there and made future trips and caught those attractions then. I think that the majority of average guests who have heard or read about Fantasmic will see that it is unavailable, shrug it off and go have fun somewhere else and really won't dwell too much about it. Even for those who have taken the time to plan out their trip and watch videos and read books (who I believe are the minority, although I could be wrong), and therefore know what Fantasmic is and look forward to seeing it, aren't gonna be all that shaken to find out that it's not available. Ya, they may get ticked off like I might about not being able to ride a particular ride on a particular trip (due to a refurb, rain, or huge crowds, or whatever), but I hardly think it's gonna ruin their trip or make them never go again. Most likely, they'll decide to go play a round of mini-golf instead, head over to Illuminations, go back to the pool, or whatever. But on the trip home, I'll bet that they still talk about how much fun they had, despite missing Fantasmic.
IMO, the only ones who are really gonna be ticked off about this and see it as the sky falling are, well...us. People who have nothing better to do with their lives than post on WDW message boards and go to WDW. :lol: Diehard geeks who go as often as they can (and yes, I'm including myself in that and mean the insult in a lightearted way, so don't take offense). People who have formed emotional attatchments to rides and shows and are therefore heartbroken to see them go...or in this case, go down to two days a week. And how many of us are gonna quit going back because of this? Not very many. Because we still have too much fun down there to call it quits on WDW because we can only watch this show on two different days now.