Renewing raspberry plants

They do get a build up virus's that attack and weaken them.
I recall reading something like every 6 years and I assume that the new should not be planted where the old were.

Having said that my first lot stayed in for more the 6 years and kept producing fine.

Not sure if the more modeern varieties are more resistant to the virus's. Half suspect that they are.

Possibly a case of watch them and if the crop seems to drop off them consider a new bed with new plants.