Once the Pens qualified ZAR, they were locked into keeping him, since he was eligible for arbitration, and he filed for it. They had to qualify him before free agency started, so not doing so meant risking losing him as a UFA and then ending up paying more for a replacement. Johansson didn't sign until over a week after free agency began. He was almost certainly asking for more than 1.5M originally.
Hindsight is 20/20, but it's not the reality of how the Hockey Ops staff would have perceived things at the time.
Also, Johansson wasn't better than ZAR last season.
20/21 season comparison:
Johansson: GP: 36
1:49 PP TOI/G
65:06 PP TOI total
15:56 TOI/G
G: 6 A: 8 P: 14
PPG: 2 PPA: 4 PPP: 6
SHG: 0 SHA: 0 SHP: 0
EVG: 4 EVA: 4 EVP: 8
Aston-Reese: GP: 45
0:03 PP TOI/G
2:08 PP TOI total
14:37 TOI/G
G: 9 A: 6 P: 15
PPG: 0 PPA: 0 PPP: 0
SHG: 1 SHA: 0 SHP: 1
EVG: 8 EVA: 6 EVP: 14
Johansson got regular power play time, albeit on the second unit. Most of his production came on the power play.
Aston-Reese barely got more power play time over the entire season than Johansson would get in a single game.
At even strength, ZAR outpaced Johansson. Even accounting for playing more games, Johansson scored at 0.22 even-strength pts/g, while ZAR scored at 0.31 even strength pts/g. ZAR beat out Johansson's total goal production solely at even strength.
Also, while Aston-Reese's career high was 17 points in 43 games in the 18/19 season, he wasn't that far behind that this year with 15 points in 45 games, so it's not like he's showing signs of dropping off a cliff. He paces at over 10 goals per season over an 82 game season, which is reasonable third-line production (his career average puts him at over 13 goals/82 games, in fact). And the Pens have him split time mostly between the third and fourth lines.
Honestly, I wouldn't be heartbroken in the least if the Pens moved on from Aston-Reese, but I really don't understand the hatred he gets on these boards. What do people really expect from bottom six forwards?