Haha, "biggest issue", that's a good one. He may be currently the worst value contract on the Leafs, I'm not denying that, but it doesn't mean the contract was a mistake or that he's the "biggest issue".
However, my overall point is you're not operating in the real world. Hometowns don't turn down the biggest UFA to hit the market in several years for a price less than others are offering. Tavares is easily one of the top UFAs available in the last two decades, the idea of a Leafs GM coming out and saying "yeah, we could have had him for less than market, but we really lowballed him so he walked" just doesn't hold water.
No GM turns him down, period. The only one in any sport I can think of that might have the stones to turn away someone in that position is Belichick, if he believed it made sense. Even then, he wouldn't be immune to pressure from his ownership to sign him for off-ice reasons.
The only comparable I can think of in all of sports is the year the Rays made the series after basically turning down the option to pick up Barry Bonds. Even that isn't really analogous, as it was more about the media circus and distractions.
And it's easy to cry over losing Kadri now that he won a cup, but he was coming off another playoff series where he badly cost the team with undisciplined, emotional play, and it's not like the Leafs just lost the guy, he was traded for Kerfoot/Barrie, with Barrie's return being viewed favorably by most analysts.
