Düsseldorf is the principal airport in Nordrheinwestfalen, and at the centre of Germany's industrial powerhouse.
As such, independent of any Lufthansa / Eurowings / Germanwings hub, a city such as Bristol, and it's hinterland should easily be able to support at least minimum once daily business orientated service to DUS, though clearly a twice daily morning and evening weekday service would be preferable for business. Other major UK regional airports support a multi daily DUS service, and have done for many years.
Still, from little acorns, large oak trees grow.