It's extremely well-designed and extensible, there is no reason to reinvent the wheel when so much time and effort has been put into it.
They will complain of things like "eventually you will have to start over with a custom solution anyway"... but whatever gripes they have, could just be put into improving the admin to make it better at whatever they're worried about.
Personally I've not run into something I couldn't make work in the admin without having to start over. My own usecases have been CRUD for backoffice users/management and I've had great success with that at several different companies over the last ~15 years.
People will say "it's only for admins you trust" yet it has very extensive permissions and form/model validation systems heavily used in the admin and elsewhere, and they are easily extensible.