Sampras did lose motivation towards later years I believe, but there is no doubt that his backhand degenerated too at the end of his career. He was reaching the late 20s and his foot work and speed was clearly slowing down (Federer is now also suffering the same fate but the guy has more weapons and effortless footwork to back it up). Given a good day, sure he can still pull off some good shots from the wing but compare to his early age there's absolutely no comparison.
Federer is stll only 28 so has at least 2 years left at the top. Sampras at age 28 was actually at his peak. His 24 match winnning streak in summer 1999 saw him play some of his best Tennis of his career and his movement was superb. Then he injured his back before the US Open and that was the beginning of the end. In 2000 he still played some great stuff but only won 2 tornaments so he wasn't as consistent as before.
His real troubes bakhand wise came in 2001 when he was 29 going on 30 yeas of age. But when he was 27 and 28 his backhand and movement was still very good.