I think they have 4 good D men at the moment, but I don't think that they have a true #1. Karlsson has flaws to his game, but I firmly believe he is a better all-around #1 D man than Chabot. He did just win the Norris.
But more specifically, Ottawa wasn't all that strong at generating offense from the back end last year. Chabot led the group with 41 total points, which was tied for 30th in the NHL. Karlsson had more goals than Ottawa's 4 highest scoring D men combined, more points than their 3 highest scoring D men combined, and more even strength points than Ottawa's 4 highest scoring D men combined. Chychrun's offensive ability improves them offensively, but they still aren't anything close to the offensive group they would be by swapping Chabot for Karlsson. Adding Karlsson probably doubles up their defensive production, which is a sizeable difference.
It also evens out the handedness of their top 4. They'd go from 3 LHD and 1 RHD to 2 and 2. Chychrun can play on his off side, but I believe he is better on the left side.
I think Ottawa would have good reason to prefer keeping Chabot since he is much younger and should age better over the next 4 years. But I don't see much argument that swapping out Karlsson for Chabot doesn't improve their team right now. I'm a pretty big Chabot fan, but Karlsson is a future hall of famer coming off a Norris win where he incredibly went even at 5 on 5 on an awful Sharks team.
Karlsson might entertain this notion, given that Melnyk will no longer be meddling with team affairs, Stone is gone, the fans have had a few years to forgive him leaving and Ottawa is poised to become a playoff team, whereas San Jose is basically destined to finish bottom 5 again for another few years.
But I'm not sold on Ottawa feeling acquiring the age 33-36 seasons of Karlsson (regardless of his career year he just had) merits shipping out a 26 year old Chabot under contract through his prime years, whilst Karlsson would have to defy father time to give them a shot at a few post season runs.
To me, continuing with all their youth development as they're now hitting that 21-25 ages for Norris (now healthy), Tkachuk, Batherson, Stuzle, Chychrun and Pinto with Sanderson 20 and Chabot 26 right outside that range - outweighs getting both Giroux and Karlsson a primo postseason run or two at the expense of 4+ runs with the younger group.
I don't see Dorion biting on this concept - if anything he looks to trade Chabot for a ~25 year old #2/3 RD instead, to keep the same core age together while improving the team's balance on the back end. Something more like Ekblad, Theodore, Dobson, Byram, McAvoy, Fox, Sergachev or Heiskanen (ignoring target team need - just identifying better candidates for Ottawa's window).
More realistically a guy like Sam Girard or Alexander Carrier could be had to fill that 2 RD slot without paying through the nose asset wise. Both have shown offensive instinct and skill, but get held down the lineup by guys like Makar and Josi - meaning there may be more offensive potential if they weren't by default playing 2PB and with 2/3rd lines primarily.