osu!Lazer will replace stable when enough people use it to be able to slowly faze out the current osu stable (which will then become the new "Stable Fallback") Currently the only big missing feature is score submission, otherwise the two clients are very comparable. Personally I predict it'll only be another year (or two) before people start switching fully over to lazer
No? Its quite the opposite. The only updates to osu! stable are bug fixes and such, no new features will be added. osu! Lazer is constantly being updated with new features, optimizations, and fixes. Overall LAzer already has a lot more features than osu! stable, and is the main focus of the dev team from what I understand