For me, the contract concerns are simply this: is he worth $7.5 million per year, on average, over the term of the deal? If the answer is yes, it's a fair deal. If the answer is a no, it's not a great deal either for the team or for the player.
I can see him being one of the top 10 blueliners in the game at some point, absolutely, the question is whether or not he will be a top 10 player at his position next year? Because most second contracts that work out tend to end up looking like bargains, while with third contracts the overpayment happens after the Cup window. This deal puts the overpayment prior to the Cup window. I don't believe he is currently one of the top 10 defensemen in the NHL, at least not next year. And I don't believe the cap is or will be rising fast enough to offset that value in the future like it did for Tavares, Karlsson, Seguin, Hall, Ekman-Larsson, Kane and Toews coming out of their first contracts.
The biggest complaint I can have is that it looks like the team bargained against itself, because if he wanted a Doughty contract he could have had it exactly, and it would have been fair and reasonable for both sides. He gets a little overpaid for the first couple of years relative to actual play and then is underpaid in his prime, which could afford the team savings that help the team build a contending roster and raise his status as a potential Cup champion, before getting a still very lucrative third contract that likely results in him being paid fairly before tailing off and being overpaid relative to ability.
The extra $4 million over the life of the deal doesn't look like much, but it's just another one of those little things that handcuff a team's options in terms of managing the cap. It might be the difference between getting a third liner player or a second line player when the team is at its peak. When you've won a Cup or 2 and are post-window, I don't think fans mind as much paying guys for past success, but now Florida isn't saving money in the shorter term and it's not like Ekblad will give them any major discount at 29 either.
I don't fault Ekblad, he has a short window to earn major money, but this is how teams start mismanaging the cap, except that the Panthers haven't won anything yet to justify paying players what they are worth. Sounds bad but it's the reality. The best teams and the Cup winners tend to have a lot of players playing above their pay grade, and oftentimes those guys playing above their pay grade are their best players too, but Ekblad now needs to be a top-3 player at his position to do that in his case, and that's a lot to ask for out of a 19-year old.
EDIT: Took a more in-depth look at Ekblad's contract, and it's very odd because the Panthers gave a lot of ground in many key areas. There's a pretty hefty signing bonus every year of the contract, the salary jumps up and down at basically random intervals instead of a steady progression upwards as the player theoretically improves, and there's a no-movement clause the first year it's eligible.
Doughty's contract as a comparison starts at $6,000,000 in salary, slowly rising each year until it peaks at $7,650,000, has no such NMC or NTC at any point and has no signing bonus, his entire contract is based on salary earned over the season. At the time of his contract signing (not when the contract was kicking in), it was worth 10.89% of the salary cap, while Ekblad's at the time of the signing would have taken up 10.27% of the cap.
Now, I like Ekblad, but Doughty had been nominated for a Norris by the time he signed his second contract, so unless you think saving 0.62% of total cap percentage is worth giving up the signing bonuses, NMC and non-linear progression of salary then Ekblad really got a great deal here for himself. I can only imagine that Tom Rowe was drunk out of his mind because there's no other explanation for why this deal is structured the way it is, even if they were trying to make Ekblad sure of his status as the man in Florida.