IF someone was to retrofit MADL and its sensors to say a Super Bug, what advantage other than stealth would an F-35 have over that aircraft?
No much. But that's like saying if someone were to retrofit F-4 systems and missiles on a Lightning, what advantage does a Phantom have over a Lightning. First its not at all certain if F-35 systems can be successfully retrofitted to other aircraft and what is certain is that cost would be very high, maybe higher than just buying brand new F-35s in the first place.
And BTW, not even Boeing is proposing such a thing for their F-15. The latest Boeing proposal for the F-15 is NOT to fit F-35 systems on it, but to fit a compatible datalink onto it and enable it to carry even larger numbers of stand off weapons than the E version now carries. Basically, they are proposing to make the F-15 the arsenal plane USAF has been talking about for a while now to accompany the F-35 and give it more firepower.
Boeing Sees Upgunned F-15 As Potential ?Arsenal Plane? | AWIN_Defense content from Aviation Week