Frankly, OnePlus doesn't have to do much to stay 'relevant' among the general public. They just have to bring back the "OnePlus One spirit".
Seriously though, the level of excellence the phone delivered back on 2014 with the One, was outstanding. It was a true flagship killer, coming in at just about $220! Since then, and several invite systems later, OnePlus has grown. Both financially and strategically. They figured out that a short refresh of their recent popular phone could gain them profit by launching a successor during a festive season ( launch of the 3T at december).
Pretty much, they set off on a track to make money. It's pretty apparent that they've strayed a little from their motto of making phones that "never settle".
True, they still make the most hardcore spec beasts out there, with 8 gigs of RAM and top notch storage speeds. But specs are not what good smartphones are entirely made of.
They need to focus on giving the most value possible for the price tag. They've raised the price on the Oneplus 5, but have skimped on basic lifestyle features like water resistance rating and maintaining a unique design.
If the Oneplus team can get their heads together and focus entirely on optimising their already-pretty-good smartphone user experience and not focus on just "juicing up benchmarks", they can definitely be on top of their game for a long time to come without breaking a sweat.