While I think multi-page apps are a cool Retool feature, I generally prefer single page apps because they are more responsive, have breadcrumbs and create a more seamless experience for the end user.
In the "main" section of the application, I would love to define different "main sections" that I could tab through. While you could theoretically do something similar with a tabbed container, the app loses too much screen real estate.
Even if multipage was switching the "main sections" instead of entire applications, temporary states, breadcrumbs and responsiveness would improve greatly.
For example, take an instance in which a user is looking to create a standard CRUD interface that is intuitive and shows a decent amount of data in a table. Suppose they wanted to fill out a large form based off of data in that table and a couple temporary states. In a normal multi-page set up, these values have to be passed through url params for it to work properly. If the apps were based more off of these tabbable main sections, the app would be more responsive and a lot more dynamic.
In general I think a large pullback of using Retool that hasn't really been talked about that much is screen real estate limitations. With a solution like this it increases the types of applications and complexity of tools that can be created in Retool