It has to do with the expectations of guests who do not think it's worth it to wait in line -- particularly for those who made efficient use of the old Fastpass system, and will be the biggest losers under the new system.
Here's my experience: pre-Fastpass, my family showed up at rope drop, hit every headliner we could, and then waited in lines for the rest of the day. We were there during Easter breaks, so if waiting for 45 minutes or even 90 was what we had to do, that's what we did, and we didn't think twice about it. Everyone else was waiting in standby: why shouldn't we?
Fast-forward to '99 and after: combining rope drop arrivals with Fastpass, we NEVER had to wait more than 25 minutes for ANYTHING. Then we switched to off-peak times of year, and with rope drop and Fastpass we never had to wait more than 10-15 minutes for anything. We got used to that. We liked it. Waits of 20 minutes or more became something we considered to be "unreasonable." If it was more than 20 minutes, we Fastpassed it. Easy-peasy. Short waits were our new normal, and it suited us well since we were trying to cram AS MUCH AS POSSIBLE into a single 5 or 6-day trip every 2 years.
We've been spoiled: I admit it. Now WDW is telling us that for at least some of the E-tickets we want to see, we're going to have to scoot back into a standby line -- a standby line much longer than they were pre-'99, thanks to Fastpass+, and that even our standby waits for non-E-tickets will be slowed by newly-created Fastpass lines. Longer lines for all but 3 attractions every day means fewer attractions we get to see, and less value for our vacation dollars. Everybody is paying more and getting less. That's why people are up in arms.