Dahlin had 44 points. Lets be conservative and assume his development alone will net him another 4 points next season which is pretty reasonable I think considering he scored more during his second half last season.
Then add 2 more minutes per game of icetime. That will put him in the 23-24 min per game range. Thats what, like 8 games worth of icetime over a season. So lets assume another 4 points. Then more PP1 time, another 4 points. Last but not least, Buffalo seams like an improved team, not by much I guess, but with higher quality teamates we can at least expect a better +/- and maybe a couple of more points over a season. That would put Dahlin at around 55 points next season.
Remove some of the rookie mistakes and we are looking at a borderline top 10 defenseman by the end of the next season.
I know development isn't linear and that there is a lot ifs for this to become a reallity. But remember that Erik Karlsson absolutely exploded and won the Norris the season after he had a similar season to the one Dahlin just had. I don't expect Dahlin to do the same, but it shows that everything is possible for a defenseman with that kind of talent.
Worst case senario not taking any big injuries into account I think Eichel and Dahlin is top 10 center and top 10 defenseman for at least another 10 years each with a lot more upside on both players.
Hard to pass on McDavid but if there is anything the Oilers have shown is that McDavid alone wont garantee any sort of post season success. A franchise C and D will get you alot closer to the ultimate goal.