A lot of people’s response to the new Toy Story movie was some flavor of “wait, why?” and if there had been a new trilogy of films announced, there’s a good chance that actually would have quelled some fears.
Of course, if that was an intentional plan, it’s likely Pixar would have announced that at the time they announced Toy Story 4 in the first place.
A new movie gives the impression the studio is trying to keep a successful franchise alive even after its run its course, a new trilogy implies that the story actually has someplace to go and this has all been thought through.