I've been around long enough to see lots of big retailers come and go. There seems to be a natural cycle. Bring in customers with lower prices, better selection, better service and/or better quality. Put the "old" retailer out of business. Over time, the newcomer's prices drift up, quality, service and selection drift down, until another newcomer puts them out of business. Lather, rinse, repeat.
I see WM well down the path. I see higher prices in most cases, but more significantly, drastically reduced inventory. Quality and service are still pretty good in most WM stores, if not always as good as we'd like. The pivot to "lifestyle" (read: "high profit") items is probably the most telling sign of a very uncertain future.
I've seen the same thing in home improvement stores. Anybody remember HQ? Lowes and HD put them out of business with better prices, better inventory and equal quality. Just as HQ had put our own local chain, Grossman's, out of business before that.
Now I'm seeing Lowe's and HD pivoting toward high-profit "lifestyle" lines, expanding their design studios and lines of high-end plumbing fixtures, at the expense of ordinary must-have inventory on the lower end of the profit spectrum. My expectation is that in a few years, some smaller home-supply outfit will target the ordinary folks just looking to build and fix ordinary stuff, and give the big guys some real competition.
Amazon, likewise, has taken most of my business from other on-line retailers through fast, honest shipping policies, great selection, good prices and great customer service. Honest customer reviews and Q&As are a huge value to me. eBay has gone the other way, to the point that I avoid them for anything but a last resort. No doubt, in five years this too will change.