Tarl Wawrick is making a case for that in the video linked below. He makes a lot of good points.
I have been thinking along the same lines too, even if for slightly different reasons. Just like Wawrick I believe the current FBI to be highly corrupt. Corruption, naturally, makes it inefficient - for example, I tried to request their help twice, in 2000 (or 2001) and in 2015 - and never so much as got a response. And my complaints were legitimate cases of cyber crime - specifically, internet impersonation.
But my thinking on the issue goes like this. Let us say we disband them. A message immediately goes out there - you though that a a fed you have job security for life, but you don't. It will literally work to help select different people for government jobs - self select, if you will, as people will start to apply for these jobs for different reasons.
I imagine if we were to disband this agency, some functions of it - genuine, beneficial to society functions of that agency - will suffer a disruption. But that slack will be picked up by other parties, public or private. Meanwhile, we will start to refashion the very concept of government and reap benefits far outweighing the negative fallout caused by possible disruptions.
References
The FBI is a Threat to American Democracy and a Derelict Relic
Tarl Wawrick, 17 August 2022