I don't follow the rest of the league that closely TBH so I guess I'll take your word for it. Not sure how Marner is viewed around the league but I'd guess he's viewed as at least a top 20 or so player in the league, and one that is very entertaining and will help sell tickets. He's also under contract for four more years (prime years) for a bargain cash wise so to a team that values real dollars over cap space, that adds to his value. He's also young, coming off a 1st all-star season and just entering his prime so I'd be surprised if those other players measured up to that, value wise.
Just focusing on Carter/Richards,
they were 25 at the time of the deals, so also young/just entering their prime
contractually they were on the current equivalent of ~7.3m x 8
Over the previous 3 seasons Carter was 6th in the league in goals, Richards 28th, they were 25th/26th in points
Carter had a bit of a playoff ghost rep, but Richards was seen as the posterboy playoff captain warrior, fresh off a gold medal with Canada at the olympics.
He (Marner) is definitely a higher profile producer, but I think the playstyles and league preference close the gap enough.
Nash there's no real statistical case to be made. The NHL had a love affair with his size and skillset and almost unanimously decided that his production was being brought down in Columbus.
I don't think any of the 3 templates would be outlandish next off season
Nash- Future 1st+ 1st round pick prospect + Top 6 forward + Young top 9 forward
Richards - top 2-3 prospect in hockey + young top 9 forward with rare skillset
Carter- Top 10 pick, young former top 10 pick producing as a top 6 player