My 30-year-old Korean Spice Vibernum is dying
A few weeks ago I went out to trim what I thought were a few dead branches on my viburnum and found huge sections dead or dying. My question is whether the bush is at the end of its life span or if it is being affected by the viburnum crown borer.
While we do not know the expected lifespan of Koreanspice Viburnum, the dieback could be tied to environmental stress that resulted in opportunistic infection. A canker disease called Botryosphaeria is responsible for localized branch death on shrubs it infects, and drought- and heat-stressed plants are common vulnerable targets. Viburnum is known to contract this disease. Wood-boring insects like the crown borer moths you mention also tend to target stressed and weakened/injured shrubs, though differentiating between the two ailments will be fairly straightforward once the dead wood is removed. It's also possible the two are occurring concurrently or sequentially. If any tunneling, frass (insect poop), and sawdust is seen at the point where damaged wood meets healthy wood, borers are or were present. Fungal infection causes sunken and sometimes discolored bark, cracking, and usually discoloration/staining of the wood itself.
https://extension.umd.edu/hgic/topics/botryosphaeria-canker-shrubs (while written primarily for Rhododendrons, the disease information applies to other species as well)
The groundcovers at the shrub's base appear to be two quite aggressive, vigorous, and unfortunately invasive species - Periwinkle (Vinca minor) and English Ivy (Hedera helix). Their competition with the roots of the Viburnum for moisture and nutrients may have been tolerable to the plant when the weather was cooperative, but in times of heat or drought stress, this can be quite detrimental. Even if both co-existed for years prior to these symptoms, the shrub could have been experiencing sub-lethal stress that only recently reached a tipping-point. The overly-wet weather in the springs of 2018 and 2019 likely stressed or injured roots (Koreanspice Viburnum not being a wet-tolerant species), and the following 2019 autumn drought also impeded any root recovery. We experienced one of the hottest Julys on record this year; similar conditions occurred in recent summers as well. If you give the planting bed supplemental irrigation during hot, dry periods, that is good and may have been sufficient to stave-off issues up until recently.
All you can do about either culprit - fungal dieback or borer dieback - is to cut out all dead wood, as low as you can go to where it meets healthy wood (or close to the soil line, whichever is apt). Treatments for either are ineffective at this stage. Keep the plant in as good a condition going forward as you can, which essentially means watering as needed (guidelines in link below), not watering when it's not needed, and consider replacing the groundcover with less-rampant alternatives (second link below).