If Columbus gets that much better and if you are fans of any eastern conference team, why would you cheer for this? It will make that much tougher to beat them in the playoff and they already are a playoff team that can be very competitive comes playoff time as it is now?
As for Isles, yes it losing Barzal or Pulock may make their team worse but I think Isles, as it is right now, is not good enough to win the SC without adding a legit sniper. On the other hand, Columbus with Barzal, unless they either end up losing Dubois or may lose Jones down the road as a result of this offer sheet, likely will win the SC within the next two years. So if you are fans of any contending team, especially in the east, close to battle for an SC in the next 2 or 3 years (like TML, Carolina, Boston, Caps, Flyers, maybe even Pens and Tampa again), you would not hope Columbus is the team that will land Barzal unless you hope your team will target Dubois this fall or go after one their top Ds when they turn UFA.
Say If Isles get much worse without Barzal they will eventually go the rebuild route or retool route and in 5 years they will get back being competitive (as that is how long the rebuild will take normally - see Vancouver and Rags) and they will eventually land some young elite forwards. With the lottery draft system, the rebuild can be faster if you are a team just missing the playoffs. I will strongly recommend that the Isles hang on to all their top recent picks and prospects until they know if Barzal is resigned. They can't gut the future without a clear picture of which direction they may go in one or two years.
Most likely though Isles will match and will trade someone from their roster to make room for keeping Pulock. Failing to do that, they will trade Pulock to a western conference contending team for possibly either a good young prospect or 1st round pick. If he is on the market, there will be a lot of interest.