As much as I prefer real native applications and like hating on electron, I have to give it some credit. It is not just chromium, it is chromium+nodejs and all the stuff that comes with it.
The answer to "would it be feasible/easy to port an Electron app to a pure browser app" is no. Unless all of the communication between the frontend part and the native tooling is done via sockets.
I agree that native apps should be sandboxed, however sandboxing something like an IDE will be quite difficult. Even Apple does not sandbox Xcode.
Some data is not suited to go through asynchronous streams with a lot of parsing. At some point I needed to pass real-time eeg data to an application and we were exploring Cordova as an option. The jitter and absurd latency (over 15ms) was a non-starter. Things might have gone better but not everything can be solved by immutable data structures.
For comparison, the same data transfer using Electron native plugin took well under 1ms.
I seriously doubt it would be impossible to use Electron as it really is, a Web App.