It’s not that Perron will decline so rapidly (though he could, and probably won’t put up the same number away from ROR). The expansion team needs assets out of their draft. Taking a guy who is UFA in one year is foolish. Are they trying to win the Cup in year one? (That question only looks funny because of the ridiculous Vegas experience.) They should be building for a 3-4 year ramp up. Perron could jettison in 1 year, and if they could sign him as UFA they could also do that in 1 year for free.
I would definitely protect Sundqvist over Perron. Ideally I’d like to protect both.
The answer to your question is that they will almost certainly be looking to build a contender in year 1. The Vegas expansion demonstrably proved that these expansion rules can make Seattle a contender in year 1. Given that history, it is very likely that Seattle will in fact be trying to build a contender out of the gate, especially since they are in a more competitive sports market than Vegas was. Being good right away will be extremely beneficial from a business standpoint. And since every team is bleeding money this year, there will probably be even more "really good, but overpriced" players exposed as teams want to use expansion to shed dollars. It is highly likely Seattle will be trying to build as good a team as possible for year 1.
But even if they take a long term view, Perron is still the smartest asset to take. They would easily get a 1st + a good prospect for Perron by trading him and retaining 50% of his salary. Perron has scored 187 points in 214 regular season games since he was selected by Vegas in 2017. That's a 71 point pace over 3+ seasons and is 59th in the league for that stretch. He has been to the Cup final in 2 of the last 3 years and won a Cup with us. 34 points in 50 playoff games over that stretch (55 point pace). He's a top-line caliber forward and he plays like a pest in the playoffs. At 50% retention, his cap hit for the receiving team would be just $2M with $1.7M of real dollars owed. Most the league will be in a cap squeeze next season and teams that want to add for a run will have a tough time making money work. Seattle would get an absurd trade package for him from a contender looking to bolster the offense. His trade value as a 1 year rental (post expansion draft) is worth substantially than whatever else might be available from our roster.
Whether they would want to keep him as a 1 year option, keep him and re-sign him or flip him for assets, Perron would be the clear choice for Seattle over guys like Barby, Sunny, Dunn, etc.
I'd be shocked if we expose Perron. Berube has him glued to our captain, he's scored at a consistent 65-75 point pace in the regular season in that role, and he has 25 points in 35 playoff games in that role. He clearly loves it here, he took a hefty discount to come here and it seems reasonable to think that he will be re-signed for 1-2 years at a fair AAV when the current contract expires. I don't think we are going to prioritize youth/depth over a clear cut top line forward just because he is 33.