If you want everything to match, replace everything. Otherwise, you will always be chasing trying to immediately match the existing as other pieces of siding degrade in the future.
Painting sounds like an option, but in this case, we're talking about painting old wood some of which is reaching the end of its lifecycle. That means in a few years trying to match weathered paint instead of weathered wood...so probably repainting again.
One alternative is to relocate existing weathered pieces from a less visible location to any prominent location and to install the new wood in the less visible location. It makes the job slightly larger, but still smaller than replacing everything, painting, or artificially aging the new material.
As a note, the concrete slabs/pavers adjacent to the siding is probably contributing to the speed at which the siding degrades because water dripping off the roof will have more 'splash energy' after hitting the hard surface.