I don't understand when people say F-16 is an 80's aircraft designed in 70's and is now obsolete or about to be obsolete. I breakdown a fighter's capabilities in three categories, hard power, soft power, and killing power.
Hard Power (the visible jet): Lets leave the stealth designs out for the moment. Comparing F-16's aerodynamically unstable but sleek design, low RCS, agility, maneuverability; combat load, range, endurance, G rating, etc. with other "modern" fighters of the same class like Typhon, Rafael, Grippen, MIG29, and J10, I don't see any thing missing or severely lacking to consider it "obsolete or about to become obsolete" in hard power. Subject to driver skills, It can still match / outclass any of the mentioned and some ... platforms any day of the year.
Soft Power: According to my classification, net working, avionics, detection, targeting, ECM, and computing power etc., fall in this category. Evolution in this area (barring stealth tech sensor fusion) has nothing to do with the airframe design of 60's or 70's, which itself has evolved per requirements.
Even in this area, F-16 has not been stagnant, in fact it has been at the forefront of receiving most upgrades in almost every aspect be it radar upgrades, networking, jamming, targeting or whatever. no other platform comes close to vipers in number of upgrades and today despite being a four decade old fighter it is still very potent, battle proven beyond any doubts and has on many occasions outclassed other contemporary fighters in exercises. Any one who says F-16 is 80's technology, is misinformed.
Killing/destructive Power: This depends on he armament and the flexibility in the soft systems to integrate. With todays advancement in computer tech, integration of western weapons on western jets is not an issue. Vipers have a wide variety of options to suit almost every mission profile. Almost all of the latest US weapons find their place on one of the hard points on vipers and they have been used very successfully in recent conflicts. I am not sure about the numbers, but I believe Vipers are the biggest singly fighter force in USAF, and because of that almost every new weapon gets integrated on them, making them ever more lethal and up to date in killing as well as soft power.
New advancements in military technology are very expensive and because the military spending across the world (specially the western world) has gone down significantly since the end of cold war, new tech is not going to be widely affordable and the fourth gen fighter would remain relevant for another two decades, I doubt that even US would be able to retire all of its F-16's (the low end fighter in 15/16 mix) in the next 10 years. I think Eagles and Falcons would remain the bulk of US air fighting machine for another decade with 22 and 35 filling the air dominance / Awe / fear factor role.