Eclipse Theia is meant to be a 1:1 replacement for VSCode, and it's a completely independent rewrite. Afaik the only part they share is the "Monaco" text editor component, but there's not really much lock-in risk there.

Eclipse are also the people who run the open-vsx extension registry.

Personally, I am very happy with Sublime Text and don't see myself using anything else any time soon. But if I did need an alternative, Theia might be my first choice.

Edit: another one that hasn't been mentioned yet is Lite-XL