What is the best way to delete all default brushes from Illustrator?

That is the brushes folder, not your New Document Profiles folder which contains the documents you need to change.

As Ton already said, you will have to edit the Nes document profiles.

And then this is where all the presets and settings reside https://www.vektorgarten.de/illustrator-settings.html

I have to warn that many brushes and other assets in Print.ai disables Trim View:

By @Chris P. Bacon

You still don't understand it.

There are just very few "defaults" in Print.ai.

Everything you can load from that library menu is in separate AI documents on your harddisk. Unless you use it in your document, it won't get into your brushes panel and thus into your document. And it surely won't be loaded into memory as long as it is just on your harddisk. Deleting all those presets might be a useful idea for whatever reasons, but a disabled Trim view is none of them.

Nearly everything you can access in Illustrator is stored document based. there are very few application based presets. Whatever causes your trim view to be disabled, it's not the default brushes. It might be the brushes you have used in that document or it might be something completely different. Since you don't provide your actual AI documents, nobody will be able to tell.

I have created a 150 MB document that just contained transparently overlayed brushes on the artboard. It lagged severely, but Trim view was available. So with yours, it may be something else.