That’s a good point @adamfuller - there’s definitely a tradeoff between the control and upgradeability of the custom bridge vs. the increased complexity and attack surface of it being custom.
(There’s actually one more thing that pushes us towards needing a custom bridge now: if we end up going with GIP-0037 we 100% need a custom bridge, to handle minting excess tokens in L1 if the escrow doesn’t have enough balance)
As for the previously-bridge GRT, users will still be able to withdraw those back to L1, as the existing bridge will keep working; albeit not through the Arbitrum UI (once we link our gateway with Arbitrum’s router). Alternatively someone (?) could set up a LP in L2 to exchange those to the new GRT, then bulk-withdraw them back to L1.