NDBs need to produce power dependent on the range required due to attenuation. As an enroute aid a lot of power was important but as a locator the power can be reduced and therefore causing less interference elsewhere. As the demand for radio frequencies increased the CAA required the power of locators to be turned down to quite a low power. The VDF indicator is a motorised unit and so signal strength is important to drive it. All the things that can cause interference also became increasingly difficult to manage: electrical storms, coastal effect, night effect, airframe static and last but not least the interference from one beacon to another. All these things always existed but were more manageable with the higher power. Dead reckoning was once the norm of course and enroute aids were just that, aids. The locator NDB was simply a cloud break aid that put you somewhere close to the place you wanted to be, the runway threshold. Plus or minus a few hundred metres being common and acceptable which is difficult to understand nowadays when a WAAS GPS will place you within 3 metres in two dimensions.
Perhaps a digital format for such a very simple pointing radio. A digital NDB enabling the receiver to discriminate between the wanted, the unwanted and noise. The ADF wouldn't need to be motorised either but rather a digital display with much improved sensitivity. I await to hear from those with radio engineering expertise. I'm very aware that I could be spouting nonsense regarding such an idea.