No, it isn't a worry. Blimey, some of my youngsters sleep like that! They may bestir themselves briefly if they hear anything of interest or if they think food is on immediate offer, but not otherwise. Your dog is old and can't be bothered even with that stimulus, as often as not. It may not feel up to moving much. If it's not in obvious pain or distress and the vet isn't worried, nor should you be though you remember the dog being more active. And, if the dog's life is coming to a peaceful end, well , that's natural too, and , let's face it, the best way to go. While I don't think that is necessarily the case, probably not, there's no harm in it.