It's going to be tougher for Sergachev to put up points as time goes on. Improvements in his game will hopefully balance that out, but scoring 40 points as a rookie, on the best offensive team in the league, while being heavily sheltered most of the team, doesn't mean a guy is a lock to score 70 at some point in his career. Serg's offense took a massive step back as the season went on, and even in the playoffs, where his D looked much better, he still wasn't putting up a ton of points.
Development is not linear across all players. Some players come in a lot closer to their finalized form. The best example of that is probably Aaron Ekblad, who started his career so physically mature that there's just not a whole lot of room to grow. I think Sergachev is similar, although I do think he'll get a little bit physically better, whereas Ekblad at 30 will likely have a very similar physical skillset to what he had at 18. Other guys need longer to develop (I put Hedman in this category) because it takes them a few years, or several years, to get comfortable in their own body. So, while I do think Sergachev will improve, I think most of those improvements will be on the mental side. I don't think he's going to get a whole lot stronger or faster, or develop a better shot than he already has. His improvements will be almost all in the decision-making department, IMO, which will make him a much better player, but I'm not sure it'll lead to a whole lot more offense than we're seeing now. I think he's going to crack 50 points a few times, but 60 might be a stretch, because:
One big problem for him, in terms of individual success, is he's always going to be second choice to Victor Hedman. Hedman at 27 is still taking steps forward, which isn't uncommon with defensemen. Obviously, he'll slow down at some point, but will Sergachev at 25 be better than Hedman at 33? I'm not sold on that. The very best players in the NHL, and I think we'd all agree Hedman is one of those, who skate well, and Hedman obviously does that, tend to play at a high level even as they decline. So, for instance, top unit PP time is going to be hard to come by for Sergachev for quite some time, and who knows what the team will look like by the time he finally does pass Hedman on the depth chart?
Sergachev has a fantastic skillset, and he's pretty much perfect for what we need long-term. But what we need long-term is pretty much a guy who'll play Robin to Victor Hedman's Batman.