When they had ticket books did they have a separate admission? Are you saying they would love to go to a system with a fat admission plus purchased ticket books (or a modern day equivilant).
Originally you had to buy a book with all the tickets which also granted admission. The rides also had a admit price you could pay cash to enter. That's DL's model, but you still needed a park admit ticket. It had a value of its own and I -think- could be purchased all by itself. Some rides had no tickets because they were sponsored. Adventure Thru Innerspace, for example. That's why Adventure Thru Innerspace is so widely known in nostalgia land.. it was free while not much else was. It's not that it was a particularly grand attraction.
Around the time of MK it was, if i recall, ticket books (book included admission ticket and transportation ticket for monorail) and the ability to buy tickets without the admission in groupings or even trade up lower level tickets to the higher level tickets.
When Epcot was preparing to open, there was even an included ticket to ride the monorail to Epcot's preview center at the Epcot monorail station - wander around the front gate of the park - and ride back to the TTC. Wonderful way to promote the park. When Epcot opened everything was sponsored.. so why have tickets?
They applied the same logic to the MK. Anyone who showed up with a book got an all-ride ticket swapped for it. This is also when the mag-strip tickets came into existence. Epcot's turnstiles were the first to have them. Once you were in, you were in and everything was open.
Epcot always operated this way, so there's no real data about how a A-B-C-D-E ticket system would alter guest patterns in the park. Magic Kingdom obviously did not.. and it's clear the crowd patterns changed. Smaller attractions lagged, and there became concerns over how to spread guests out.
Long lines in general helped spread people out to an extent. A long line would get people to not bother while they'd flock to short waits. But with the same number of people in a park...
Ticket Books: Moderate waits max at any attraction, attractions utilized at roughly the same rate across the park.
All-Ride: Long lines at e-tickets, moderate or short waits at other attractions.
Then in the late 90's FastPass was introduced to address "the top complaint of park guests" - "long lines." (Remember lines were never really long in the ticket book days.. There's a reason rides like Haunted Mansion and Jungle Cruise hastily constructed queue extensions.)
And that exacerbated the schism between e-tickets and other rides...
FP: Long long lines at e-tickets, ones that don't move much.. and short waits at other attractions.. several now walk-ons that had never been previously.
Disney knows this, the industry knows this (it's why Universal got rid of their FP for day-guests without the up-charge) and they want the old way back. They want guests spread out around the park more evenly.