All very true Tsits's best prospect is to develop a serve better than Federer and Sampras (sounds all too much like Roddick) and couple that with Sampras clutch returning. The issue for Stefanos is he may be clutch on return like Sampras, but his overall stats are very likely to be on par with Roddick. Stefanos doesn't have a game that feels like Roddick's, but the numbers say he'd be lucky to have Roddick's prime stats.
All you say is true, but a bigger serve for Tsitsipas might get him the upset of the Mad Lad there and there. If he's able to hold serve against Medvedev he's very much in the match.
Goatsipas just turned 21 so we don't know for sure his ultimate return level, but we can be assured he's no Djokoray returner or even a Mederer at this point.