I think its arguable which actually drove the other. Indifferent service and knowledge free staff were already a well established feature of the high street long before online sales had begun to really bite. By the time online really got running there was little love left for most high streets, except for those whose planning commitees took a longer term view on shaping their towns when it came to applications.
Jessops was always my bellweather, simply because I bought my film and paper stock there once a week or so. By the mid nineties the consumer section of the New Oxford street store had pretty much lost the plot and descended to Dixons level, although two or three of the pro staff downstairs were excellent till even they left 10 years ago or so. A real shame, because in the past staff there had perfomed minor miracles for me with pushing repairs and bulk orders. Good photographic shops do persevere, but they're certainly not chains.
By the time Amazon had started to really make a mark beyond books there were few compelling reasons left to show any loyalty to anything but small local shops, but they weren't immune either. Our previously thriving local fruit and veg market diappeared almost overnight with the arrival of a third large (and largely unwanted) supermarket close enough to take custom, but far enough away to make visiting both too much for most people.
I'm not sure it would eventually have ended any other way, but the 80s move to deciding that anybody could sell anything given a shiny logo, pushy tactics and a script just meant that online retail didn't have to push very hard to gain acceptance, because consumers didn't see there was much to lose.