Back in the day, shuffling party-goers into either Tomorrowland or, some years, Frontierland worked GREAT for us. Day guests were cleared from the rest of the park between 6 and 7 PM. LTT used to have the characters dressed in Halloween costumes after 6 and getting ADRs for dinner on those nights was like getting BOG is now. LOL It was great to have dinner from 6-7 and exit into a totally transformed (and quite empty) park. Or you could ride the rides in whatever land you were being 'held' in with relatively low lines until 7. We always loved it.
However, I don't see that being easy to do now. That was before they brought out the 'big' characters like 7 dwarves, Jack and Sally, or even Captain Jack (who was the huuuuge thing for a while and had very very long lines). The dash to those lines if they opened right at 7 would be CRAZY.
Pure and simple, Disney is selling waaay more tickets to these parties in order to meet demand. Clearly they started allowing day guests to stay until 7 to take advantage of the extra shopping/dining $$ those guests spend in that hour. I, personally, dislike that the start of the party is marred by this 'clearing' procedure - I just don't like the atmosphere of it myself - we have witnessed upset children being ushered out of party character lines crying - people being escorted out of the park, etc. I know they NEED TO GO, but I'm saying I miss the days when I didn't have to witness it and it wasn't a negative party of my party experience during the first hour. When the park reopened at 7 and you were released from the 'holding' land, it was so exciting and CALM and FUN.
Perhaps with the great increase since then in numbers, they could use 2 lands for 'holding' party goers from 6-7 and put the 'big' characters there to let people start lining up for them. But there are also very increased #s of guests who now enter at 4, so don't know if that many guests can even fit into 2 lands - but Disney knows. This is hardly new to them - IMO they are always pushing the limits to see how many guests they can squeeze in before they reach critical mass. Oh well, we all know Disney will not do anything unless ticket sales stop to drop - and that's not happening in great enough #s to affect them financially.