In terms of frequency resolution, you are correct. Sampling rate does not affect the resolution in the frequency domain, rather signal duration does (as you noted). Time duration is to frequency resolution what sampling rate is to frequency duration (bandwidth).

As you're probably aware, one way to interpolate the spectrum is to zero pad it before transforming it, though it's important to note that zero padding does NOT fundamentally change the resolution (it only upsamples the spectrum, without adding any information). 

I can't think of many benefits to significantly oversampling a signal, beyond a sufficient amount to account for filter rolloff. I suppose it might make the spectrum estimation problem more over-determined, but I think that would be a pretty marginal benefit in most cases. In addition, there is of course a significant cost to oversampling in terms of memory and processing speed for practical applications. 

## Edit: ##
@AlexTP made a good point in the comments that an additional benefit of oversampling is reduction in quantization error.