Of course, how FP+ will eventually affect the number of people arriving at park opening is somewhere between pure speculation and an educated guess on everyone's part.
But, I think there are a number of things that you are overstating.
First, arriving early has ALWAYS been far and away the single most stated piece of advice from guidebooks and websites. And yet, a very high percentage of guests simply can't or won't get up early on their vacations and, even if they think that they will get there at opening, they take longer to get the party ready than they think and end up arriving later.
Second, I think you are overestimating the percentage of guests that employ that much strategy for touring the parks at all. Many guests don't arrive early because they don't want to, but there are probably that many more who don't realize how much of an advantage there is to arriving early. If they see that the park is open from 9 AM to 10 PM, they think that arriving at 11 AM IS early because that will still give them 11 hours in the park.
Third, while there may be some people who wouldn't have come at RD before who may suddenly be inspired to do so because of early FP+ reservations, or because they suddenly came to the realization that arriving at RD is advantageous, that number will be offset to some extent by guests who may have come early before but are now willing to use their later FP+ to take their time getting to the park. I'm not sure which of those forces will be stronger.
Fourth, if someone has a FP+ for that first hour, they may be inclined to arrive only within that first hour, and not at the beginning of the hour. Using DHS as the example if they continue opening 15 minutes or more before the stated time, the people who are there at opening and walk directly to TSMM will be on and off the ride by 9 AM before very many, if any, FP riders have arrived.
Finally, 6000 people at any park when the gates open sounds awfully high to me. I don't know exactly how many lines there are to get into the park, but it seems like it is less than 20. At 300 (or more) people in a line of single and double file, those lines would go way past the ticket booths and probably go past the bag check lines. I think they would open the gates to let people in before they would allow lines to extend back that far. If that many people start arriving that early, maybe the result would be more and earlier openings of the parks before the stated opening times. That would reestablish some gap between real park opening and the first FP return times.
I used a lot of words to get here, but I put this into the category of "I'll wait and see what happens and adapt to it if there is a dramatic change". I don't think Disney wants that kind of a crush at opening and, if it starts to happen, I suspect that they would do something about it.