This may purely be a semantics thing, but I think it's too soon to say whether McGroarty and Koivunen are or aren't guys you can build around. Koivunen is 21 and has 20 goals and 53 points in 58 games this year as a rookie, which is a better rookie season than numerous 1st rounders have had in the past few years. Just as an an example, but Koivunen's rookie year is comparable to Peterka's AHL rookie year from 2021-2022 with 28 goals and 68 points in 70 games. If Koivunen ends up a Peterka caliber player in the NHL, that's absolutely a core player IMO.
McGroarty is in a similar situation, his production has really exploded after a slow start (12 points in 25 games in 2024, 26 points in 31 games in 2025) and is still only 20. Both guys could reasonably become 50-60 point top-6 wingers, which are more on the Kunitz/Neal type of core pieces but they're still core pieces.
Now if you mean "they won't become Crosby or Malkin caliber guys", then yeah I think that is fairly obvious. But I'm not seeing anything that prohibits them (plus Brunicke, for that matter) from developing into contending caliber core pieces. They just won't be the focal points of those core, they'll be the Kunitz/Neal/Orpik part of the core. They just need to pull off probably the 1 Malkin and 2 Letangs and Guentzels to complete that core with their next few high 1sts.
This is a huge if, but if you can get those Malkins, Letangs and Guentzels with those upcoming 1sts, I feel like a core of Malkin, Letang, Guentzel, Kunitz, Neal and Orpik is a contender caliber core. If you win the lottery this year, Schaefer immediately becomes that new Letang, but you probably need another lottery win or to strike gold on a pick to get the Malkin and Guentzel.