Morgoth would have died if
Fingolfin had stabbed Morgoth's heart, as
Turin's gurthang had stabbed into Morgoth's heart in
Dagor Dagorath.
You’re conflating what
Tolkien wrote about the confrontation between Morgoth and Fingolfin, which is (by conceit of the tale) an event in
Arda’s past, with what you presume to be an event in Arda’s future.
In section VII of “Myths Transformed”, part (ii), in
Morgoth’s Ring, Tolkien wrote,
Melkor “incarnated” himself … permanently. He did this so as to control the
hroa, the “flesh” or physical matter of Arda. … A vaster and more perilous procedure, though of similar sort to the operations of
Sauron with the Rings. Thus, outside
the Blessed Realm, all “matter” was likely to have a “Melkor ingredient”…
But in this way Morgoth lost … the greater part of his original “angelic” powers … while gaining a terrible grip upon the physical world. … The whole of “
Middle-earth” was Morgoth’s Ring… War against him might well end in reducing all Middle-earth to chaos, possibly even all Arda. … [T]he final eradication of Sauron … was achievable by the destruction of
the Ring. No such eradication of Morgoth was possible, since this required the complete disintegration of the “matter” of Arda…
Regarding Morgoth’s strength, in part VI of “Myths Transformed”, we find that in their war against Morgoth in which the
Valar overthrow Morgoth in his
fortress of
Utumno to capture and imprison Morgoth in
Mandos for the protection of the newly-awakened
Elves,
[The Valar] come at last to Utumno … and find that “the Morgoth” has no longer for the moment sufficient “force” … to shield himself from direct personal contact.
Manwë at last faces Melkor again, as he has not done since he entered Arda. Both are amazed: Manwë to perceive the
decrease in Melkor as a
person, Melkor to perceive this also from his own point of view: he has now less personal force than Manwë, and can no longer daunt him with his gaze.
Either Manwë must tell him so or he must himself suddenly realize (or both) that this has happened: he is “dispersed”. But the lust to have
creatures under him … has become habitual and necessary to Melkor, so that even if the process was reversible (as it possibly was by absolute and unfeigned self-abasement and repentance only), he cannot bring himself to do it.
When he fought Fingolfin, Morgoth was weaker still, having “dispersed” even more of himself; but he was still greater than Sauron or any others of the
Maiar. Whether or not Fingolfin could “kill” him, unhouse him by destroying his physical form, is purely speculative. In Tolkien’s mythology,
Tulkas once more seizes him in
Angband at the end of the
War of Wrath, and the Host of the Valar returns him to
Valinor. In Section VII part (iii),
[Morgoth] was judged, and eventually taken out of the Blessed Realm and executed: that is killed like one of the Incarnates. … [T]hough he had “disseminated” his power … far and wide into the matter of Arda, he had lost direct control of this, and all that “he”, as a surviving remnant of integral being, retained as “himself” and under control was the terribly shrunken and reduced spirit that inhabited his self-imposed (but now beloved) body. When that body was destroyed he was weak and utterly “houseless”, and for that time at a loss and “unanchored” as it were. We read that he was then thrust out into the Void.
Again, maybe Fingolfin could have achieved this or not: certainly Morgoth understood that he could be injured, as indeed he was, and was quite possibly afraid he could be “killed” by the Elf-king; but whether or not Fingolfin possessed sufficient means to do this is most unclear.