As a (forced) recent buyer of a new cellphone (my cell company merged with another and shut down their own legacy system), I'd say the downturn in cellphone sales is a *combination* of both lack of innovation and perceived value.
The 2 major cellphone manufacturers, Samsung and Apple, are putting the vast majority of their sales & development energies into $1000 / £1000+ high-end cellphones. And not everyone WANTS or can spend $1000+ on a new cellphone every, say, 3 years.
I certainly didn't, I simply don't see a good ROI there. But those same cellphone companies utterly and completely FAIL on good alternatives (here in the U.S., anyway) - from the $1000+ tier, in which you have many choices, the next tier with as many choices is...the $400 or so bottom tier. I was offered at least 6 carrier-rebated items at that tier but none had the features and build quality I needed.
So what about the middle, the $600 tier? Crickets. 1 current choice from the carrier(s), with "performance" that doesn't justify the cost (the Samsung A53, which not much better performance than the lower-cost A32).
I was pissed. The company was making me upgrade, shutting down compatibility with my current phone that I was happy with (LG V30+ 128GB) but yet their offers in replacements...sucked.
After much comparison shopping and hand wringing over the poor choices, I bought an open-box LG Wing off eBay (take THAT, TMob!)
With the duopoly of Samsung and Apple giving us "choices", most of us see no value in updating to the latest and "greatest" because they either really aren't all that different, blatantly overpriced, or both. If these idiot companies would make mid-priced models with the features *most* people want, forget 16 cameras et al, I think more people would be willing to make purchases. But not at $1200 for something great, or $400 for basic, with not much enticing in-between.