IMHO, the "advantage" of getting to the park at opening is greater than the "advantage" of being able to make FP reservations in advance.
The guests who get their first 3 FPs same day or the day before will obviously have fewer choices than the guests who make them earlier. But, from my own experience in August and November (generally crowd level 5 and 6 type days) FPs were available for several rides that I would consider "headliners", just not for the very most popular rides like RNRC, TSMM, Soarin, TT, and 7DMT. Availability will presumably be lower at the very busiest times of year like Christmas and the peak of spring break, but I haven't seen many, if any, reports that there are no headliners available same day. But then, I consider attractions like the mountains at MK, POC, HM, Mission Space, Tower of Terror, the Safari, EE, and Dinosaur to be headliners.
The best solution for guests who can't make their FP reservations in advance is to get to the park early and do the attractions for which same day FPs are not available. Then you will still have your 3 FPs to use later when the crowds get larger. It is also a good idea to avoid parks that have AM EMH unless you are an onsite guest who can take advantage of them. Of course, during the holidays there is often no way for an offsite guest to do this at the MK because there are usually AM EMH every day. But, when those EMH begin at 7 AM, an offsite guest can still accomplish a lot by getting there ready to enter at 8.
A guest who arrives at opening and uses an efficient touring plan will still be able to do more than the guest who arrives later or tours inefficiently or both. The limitations on FP+ reservations that so many people dislike (only 3, tiering, and no more than one for any one attraction) serve the purpose of significantly mitigating the advantage that guests who make FP+ reservations in advance have over other guests.