I am no FP expert and we only go to WDW once per year.  However I am a businessman and I want to look at this topic from purely a business perspective.  If I was in charge how would I analyze the situation?
First let's look at why FP was created (I think it was around 2000).  Disney was trying to address two problems.  First, guests complained that they had to wait in long lines.  Second, Disney wanted guests to spend more in the stores.  The idea behind FP (look this up on youtube and you'll see interviews saying this) was that a guest would shop rather than standing in line. Here was the vision: a guest wants to ride Thunder Mountain.  They walk over and see a 1 hour wait time.  But there is a fastpass machine!  So the guest inserts their ticket and gets a return time an hour away.  Instead of standing in that 1 hour line they are free to go buy a snack or buy a souvenir and then return to the ride one hour later and ride without a wait.  The guest rides Thunder Mountain at the same time (1 hour away) so they are happy and they spend money on crap so Disney is happy.  Win/Win!
We now have 20 years of data on what happened with Fastpass (doesn't matter if it was paper or FP+).  I'd ask two questions from my analysis.  First, did customer complaints from 2000-2019 decrease compared to 1980-1999 (of course adjusted for attendance)?  Second, did spending increase (again based on attendance)?  If the answer to one or both of those questions is "no" then there is no way I'd bring back Fastpass in any form close to what it was.  As a businessman I realize Fastpass costs money.  You have a separate line, you have employees checking to make sure the person has a FP.  With FP+ you have technology support costs.  So if it's not making the company more money than it's costing the company it should be scrapped entirely.
Would I give up on the Fastpass idea?  No.  I'd look at the competition. What does Universal do?  What does Six Flags do?  They both have paid FP options.  I think universal has something for people who stay on site.  The Six Flags Flash Pass annual pass addition costs 4 TIMES more than the normal annual pass.  That is what I would choose for Disney if I was the CFO or Chief Parks Operations Executive.  I'd go with a FP option that makes the company the most money.  And I don't think paper FP or FP+ did that (of course I'd actually look at the data, but I personally think the data would not show any more spending or any fewer customer complaints).
		
		
	 
I would say that this is an extremely poor way to evaluate the guest experience. Customers don't usually complain when they aren't happy. They just choose to spend their money elsewhere. It doesn't happy right away either. As more and more people see the loss in value they look to competitors. If competitors can then fill that need with better value customers will switch. The only thing that keeps customers despite lower value is a personal identity with a brand. If a brand appears to be losing that identity and is no longer a champion for shared values then people are even quicker to move on.
There is more to business, in a competitive environment,  than increasing margins. People spend money on Disney merch because they want to connect with Disney's values (family, love, memories, dreaming etc).
We've seen so many businesses over the last decade, that have considered themselves indestructible, reduce value and increase margins only to be destroyed by a small upstart. (Ex Uber vs Taxi companies)
The business rules created by boomers are generally not effective any more. The most common rules in business now are to create a product that creates immense value for a lower cost to customers. Scale until you have deep market infiltration. Then create "add on" or "premium services" that cost more, have higher margins, lower value AND allow people to connect with the brand on a higher level.
This isn't accomplished by nickel and dining. Buying 1 FP at a time will create friction in the purchasing process. We will have to make too many individual purchasing decisions that will drive up stress and make it more clear in our mind about how much money we are spending at Disney. This will create a negative association. This is why magic band purchasing was created. To avoid having to pull out our CC and wallets.
Whatever Disney does, they really need to focus on the guest experience. People are not happy to pay individually for 1 ride. They may do it initially but will be annoyed. However people are willing to pay more for an experience that is more encapsulating of that dream family vacation. And they are happy with Disney for offering it.
Whatever Disney does, I hope they build it as a package that sells time, family, less stress and greater connection and not as a Carny asking for a tip so you can bypass the line.