More specifically, Walmart needs a huge source of income that isn't tied to retail or ecommerce. AWS is what brings in the $ for Amazon so they can operate so competitively in the commerce space. If WMT doesn't figure out an extra revenue stream, they will be on the eternal hamster wheel trying to keep up.
You'd think with all the talk about innovation, Walmart Labs, Store No. 8, etc., that amazing things would be happening, but no. Not really. Most of the "look what cool things Walmart is working on" stuff that gets into news and magazine articles lately, will probably never see the light of day. Or if it does, it'll be watered down and Amazon will have long since gone to market with something better.
I wouldn't count out Target either. They seem to be stepping up their game really quickly, they still have a better reputation than Walmart, and I could see them being first to market with something that doesn't rely on retail or ecommerce to make them money.