Application without modules

Yes, of course it is possible. By the way, take a look at the sample app. For example, the installer doesn't use application modules (it has its own module system for each step), neither does Jet Studio and so on.

Application modules are a thing that is offered to you, but not imposed, and it is possible to develop an application without them if appropriate.

However, for normal projects, the use of application modules is highly recommended. Particularly for large projects, separation into smaller parts is almost a necessity, but it is also useful for smaller projects. Even the php-jet.net website and this forum take advantage of application modules.

But the decision is yours. Jet is supposed to help you, not dictate to you.