It never ceases to amaze me how "fans" love to look for the slightest chink so they feel justified to rip players apart. It's like they hate to see players doing well & can't wait to pick them apart.
Kyle Walker is far from finished, it is clear to see he isn't as fit as he could beat the moment due to a series of injuries & missing games because of them. Everyone slows as they age, it is inevitable, but I don't think that time is now for Kyle, unless these injuries prevent him from regaining full fitness.
If you watch the "race" with Werner he started sprinting slightly after him & by the time W crossed he had not lost ground, just didn't gain like he normally would, so you cannot say he has lost a yard of pace just yet, & would back him to be as fast as usual by the seasons end, injury & fitness permitting.