I understand the logic: Tavares has seen the Isles miss the playoffs even with him there, so they need to upgrade the defense and goaltending.
Putting Leddy and Ekman-Larsson together on the left side of the same team might reduce each of their workload enough that OEL isn't -28 and Leddy isn't -40. But what if it doesn't? They each got about a third of their 40ish points this year on the power play, and the Islanders will only have so many power plays to go around. You're overpaying for a one year rental on a left defenseman who's got the same strengths and weaknesses as the one you already have, who isn't getting the job done.
Jake Allen won thirty games once, but not this year, and he's 27, the age most athletes hit their peak.
https://www.hockey-reference.com/players/a/allenja01.html What if it's all down hill from here.
So, while I agree that upgrading the defense and goaltending could help convince Tavares that he shouldn't just join one of the teams that's already in the playoffs without him.... I'm not entirely convinced you have done so. Probably. Slightly. Maybe?
If I were Tavares, I'd still pick Toronto, Vegas, or maybe Colorado. In this scenario he has a chance to make a plan to stay in the playoffs for the next seven years, or to hit them once or twice before OEL walks and Allen declines. Those other teams have just as much young talent, and are in the dance, or closer to it, without Tavares, than the Islanders are WITH Tavares. So unless Tavares has no actual value, adding him to them lands them in the playoffs, and they stay there a while.