What kind of security can a bus provide? CAN is as simple as it gets for its purposes - it's a very simple broadcast-only shared-media bus with prioritized messages.
If you try to replace it with Ethernet then you'll get loads of problems, starting with a requirement to have point-to-point connections between endpoints and switches and then moving on to DoS protection and priority-based transmission.
And security guarantees won't get any better - Ethernet does not guarantee anything.