Early extensions usually take into account the price of what's left on the rookie contract. Also, the going rate for players is only going to go up. What you sign him for now would be less than what it would cost 2 years from now. Worst case, it's a wash financially and we don't have to deal with bitching and threats to holdout. It's the rare case of a Jets player that you have zero worries about whether he will drag derriere after he signs.
But you can't exercise a 5th year option until a player is in his 4th year.
Meaning the soonest you'd want to extend Jamal is the start of next season. However due to the way the franchise tag works and that safety salaries are generally not the greatest, you're saving a decent chunk of cash by not extending him while dramatically improving flexibility.
As of now Jamal is under contract for cheap next year, his 5th year option would be around 10 million, and his franchise tag will likely be roughly 12 million. The highest paid safety will get around 16 million a year.
We have Jamal under team control for 3 years with an extremely affordable number. It doesn't make any sense whatsoever to extend him and suddenly add a monstrous amount of guaranteed money to a player you aren't obligated to do so. We have no idea what the future holds. Maybe the Jets get a rare opportunity to pay an elite receiver, left tackle, or pass rusher.
Regardless we have an incredible amount of flexibility with Jamal, there's zero reason to just give that up