Ha ha! So say the Russians,”one can make a house fly with a big enough engine”.....
MiG-21 is referred as "coffin" with an engine.
But from air policing purpose, nothing can get in air as fast as this thing from the moment order is given. The start up sequence is so simple and fast that F16 will be taxing by the time MiG-21 is already at 10,000ft.
That has nothing to do with the fact that plane is old, that air forces have to deal with Russian logistics (which is art in itself) etc. These air forces are not having these planes to defend their air space in case of some big war. They have a squadron or like Romania, several, and main purpose is air policing. You really do not need state of the art technology. Focus is on air defense and ground forces in the case of war. Croatia for example has 12 upgraded MiG-21, and they are well integrated into NATO air defense. Romania was patrolling the Baltics with their Lancer-R's.
So, F16 (like Romania did) is perfect replacement for this aircraft and F16 will be hottest used plane on the market as MiG-29 is not as good as MiG-21 and again, Russians.