I don't think their days are numbered. I find them to be pretty competitive price wise against the likes of Amazon, with the advantage that I can get the item right now. Like you said, that applies to supermarkets, but if for example I wanted to purchase a kettle in my local Sainsburys I don't think they have a website that lets me check out reviews and order for my local store. I'm sure they could delver something but so could Amazon.

Also, if you order before 6PM you can get same day delivery: http://www.argos.co.uk/static/ArgosPromo3/includeName/fast-t... and they cover much more of the country than Amazon PrimeNow service. It's service like this that makes me think Argos are seriously fighting back against Amazon now.