Hockey isn't all about points. Tkachuk is the kind of guy you need to win a playoff series, i would love him on the Rangers. Marner is good too but just a different style. Plus not too many teams want to take on that contract for a winger.
Marner is a great player and puts up more points but if Tkachuk becomes a 30-35 goal 35-40 assist player while doing what he does, wouldn't that be just as valuable depending on the team needs? It looks like bad value on paper and it probably is at the moment but it doesn't mean either player is bad.
I think you need to look at a trade like this from a playoff pov. Who do you think gives you a better chance at winning a tight, physical, grindy playoff series and maybe a few more losses during the season? Plus the cap savings which is an asset in itself, it's more than just "no, Marner scores 90 points, Tkachuk only scores 40."