Should a rider only be able to ride in one league then their income will of course reduce, but that does not make the leagues dependent on each other. There are many riders that currently get by only riding in one league and using their spare time to seek additional income from elsewhere. To say the Championship needs the Premiership would be the same as saying the Championship needs Tesco/Amazon/the local garage/wherever the rider may seek a secondary income.
In your example, a rider riding Championship only will make £20,000. It should be remembered that the season lasts about 6 months now, maybe 7 at a squeak. Is £20,000 for half a year's work not a decent return?
Bearing in mind that during this 6 months, the rider may only have to ride approximately once a week on average and allowing generously another day a week for bike maintenance then the £20,000 starts to sound like not too bad an amount to be able to extract from a sport on the brink of bankruptcy.
I place no blame at the door of riders on this front but both promoters and riders need to be realistic. Many riders ride one league and work hard to fund their careers through other means. Simon Lambert is a great example of this and all the credit to him.