There's yet to be a bad Toy Story movie, and I can totally imagine a universe where Toy Stories 3 and 4 were not made with the end of the series in mind and where a final act was warranted. It probably will be an adequate sequel, but when you've already done a movie that were clearly made with the intention of wrapping up the series, and then, miraculously pulled off an additional sequel wrapping it up again, a third such outing just seems foolhardy.
But, whatever, Disney doesn't make money off having a well constructed series narrative arc that we can reflect on and respect. They make money off cashing in on a series' goodwill to get people to the theater. Here in the monitize-nostalgia age every franchise is guaranteed to get at least one too many entries. Going out on a high, or even reasonably satisfying note is just leaving money on the table that could have been grabbed through another lazy sequel, reboot, etc.
And this is from Disney, who generally has a history of being much more oriented towards preserving and burnishing what they consider their legacy than every other entertainment conglomerate.