(The best attempt by far was a cloud email service called Shortwave, which was at least initially intended as a true spiritual successor to Inbox. Sadly, even that project ultimately strayed too far from the Inbox UX, implementing a non-optional 2-pane view that did away with one of Inbox’s best UX concepts.)
Given this constant stream of false starts and betrayals, I wanted to build something that looked and worked more or less exactly like the original Inbox, but was fully local and could work with any email account. Something that, once you had it on your computer, wouldn’t suddenly vanish or change because the creator lost interest or suddenly got a different idea about how your email should look!
Since it's just a Thunderbird extension, NeoInbox works with any email account, and runs completely locally. It also includes all your favorite classic Google Inbox features like a unified mail stream, bundling, trip detection, reminders, and more! Give it a try!