I don't generally use browsers of these companies, but if they break the chain of trust they would damage themselves as much as any user. I believe in the self interest of these companies and the understanding how the effectiveness of TLS is reliant on trust. I would trust Google more than Microsoft that likes compliance a little bit too much, but since both want their browsers established, any compromise to security would disqualify them immediately. The EU has ambitions for surveillance as far as I know.
They're already breaking the chain of trust. One of the EU's TSPs was dis-trusted by Mozilla after a long history of misissuance and BR violations. This isn't hypothetical; the EU is mandating the inclusion of insecure CAs.
