Google launched Inbox in 2014 as a sort of incubator for new approaches to email, but it hasn't been quite so novel in the wake of steady Gmail upgrades, including April's big redesign. Appropriately, the company is sunsetting Inbox at the end of March 2019. The company wants a "more focused approach" to email, according to product manager Matthew Izatt, and that clearly leaves Gmail as the lone survivor.
You probably won't have too much trouble weaning yourself off Inbox if you're a loyal user. Inbox-first features like Smart Reply, Smart Compose and Follow-ups are already present in Gmail. Outside of a brief retraining period, you might not lose any major functionality. For that matter, the interface has become somewhat Inbox-like since the redesign, with larger buttons and an overall friendlier look.
If there's a reservation, it's that this won't please people who liked Inbox as a dedicated place to try features that weren't present in Gmail. The main client will still have an option to try experiments, but that won't necessarily feel the same. Still, it's not the end to what Inbox represented -- especially not if Google is more adventurous with Gmail as a result.