It's not wrong, it's 4 each position, sure, but it is still 12F vs 4D. With all the multiple position eligibilities out there (hockey is the worst for this) it doesn't make sense to sort by position, as so many guys overlap. I would group all of the forwards together to get a true representation of replacement level value, and naturally you give a slight boost to guys you can slot at wing.
Assuming you roster 5D (4 starters, one bench) that's 65 defensemen. 66 is your replacement level D.
Assuming you roster 15F (12 starters, 3 bench) that's 225 forwards. 226 is your replacement level F.
The 226 F is approximately 28 points. The 66 D is approximately 26 points. So yes, you need Matthews to outscore Karlsson, and I bet that will be the case 90% of the time moving forward. Matthews is the hold here.
Even if we go with the 66 C, my metric has that coming in at 36 points, with the 61st at 38 (likely differing from yours due to position eligibility variation). I still bet that Matthews can outscore Karlsson by 10 points on a regular basis moving forward.
Plus Matthews is 8 years younger.
To me the logic and math says hold Matthews.
10 TEAM FANTRAX LEAGUE (Salary Cap)
23 Man Roster (Keep 16) + 12 Minors
G, A, PIM, SOG, STP, HIT, BLK, TK, FOW
W, SV%, GAA, SO
C: Tavares, Pettersson, Bergeron, Dubois, Trocheck, Gaudette
RW: J. Anderson, Coleman, Garland, Puljujarvi, Hoglander
LW: Tkachuck, Miller, Goodrow, Mangiapane
D: McAvoy, DeAngelo, Krug, Borowiecki, Gustafsson, Lindgren
G: Shesterkin, Samsonov
Minors: Kahkonen, Sandin, Jarvis, Rasmussen, Elvenes, Abramov, Morin, Brisson, Clague, Gilbert, Hoglander, Timmins
IR: Kucherov
10 TEAM FANTRAX LEAGUE (Salary Cap)
23 Man Roster (Keep 16) + 12 Minors
G, A, PIM, SOG, STP, HIT, BLK, TK, FOW
W, SV%, GAA, SO
C: Tavares, Pettersson, Bergeron, Dubois, Trocheck, Gaudette
RW: J. Anderson, Coleman, Garland, Puljujarvi, Hoglander
LW: Tkachuck, Miller, Goodrow, Mangiapane
D: McAvoy, DeAngelo, Krug, Borowiecki, Gustafsson, Lindgren
G: Shesterkin, Samsonov
Minors: Kahkonen, Sandin, Jarvis, Rasmussen, Elvenes, Abramov, Morin, Brisson, Clague, Gilbert, Hoglander, Timmins
IR: Kucherov