Yes. That is absolutely spontaneity. And guess what - without fastpass, you find acceptable wait times more often. And if you’re familiar with the park, you can find them even more often.
And I’ve never even been in Be Our Guest despite having been to the park at least four times since it opened because, even starting out a month ahead, I couldn’t get a reservation. That didn’t happen in the days before ADRs and, specifically, WDW staffing to the numbers of ADRs.
The idea that advance planning within the park is just naturally necessary is absolutely untrue. Fifteen years ago, no one had to reserve rides months in advance, to draw up a rigid schedule for the day. It is that way now because WDW made it that way intentionally. Once again, people are trying to absolve WDW management of all blame by pretending they have no agency.
As to this being optional - look at what Feather said. This is the first step to Disney scheduling more and more of your day, with or without your conscious knowledge. This has obviously been a big part of the plan for FP plus from day one.