A farewell concert of the Who that drew approximately 50,000 rock 'n' roll fans to Arizona State University's Sun Devil Stadium last night not only asserted the vitality of the group's music, but also exemplified what has gone wrong with rock 'n' roll.

Beneath a full moon on Halloween night, the Who dazzled a sea of clench-fisted music-lovers with nearly three hours of power chords, scissor jumps and primal screams.

The six-hour show featured John Cougar, Loverboy and The Who, an 18-year-old British supergroup that is in the midst of its farewell tour of the United States.

Cougar, the opening act, was hit on the right side of his head when a whiskey bottle was thrown onto the stage at the end of his second song. At first, it was announced Cougar would not continue. But he returned after a 20-minute intermission, wearing a bandage and a hard hat.

Authorities also reported that more than 100 fans were treated for alcohol and drug abuse.

There were, however, only scattered incidents of gate-crashing, rock-throwing and bottle-breaking. That was a relief, considering the high number of arrests made during other concerts on this farewell tour — and considering the 1979 tragedy in Cincinnati, when 11 people were trampled to death at a Who concert.

If Who fans exhibit a penchant for destruction, it probably arises out of the group's message of teenage revolution. The Who long has sung about conflict, and about the attempt to reconcile anti-authoritarian ideas, high ideals and personal demons.

When Who vocalist Roger Daltrey coiled himself around a microphone in 1966 to sing "My Generation" and to define the anger that was at the core of the group's music, there was a naive dream of building a better world with an art form. But now the Who are older and wiser, and as some of their newer music shows, they are a bit disillusioned.

"Substitute," Peter Townshend's guitar riffs slashed above the crowd, and Daltrey's vocals were loud and clear. Along with bassist John Entwistle and drummer Kenny Jones, the band wove various hits in such a way as to say, yes, the angry young man bit is fine, but now it is time to grow up.

Hits from the past included "Can't Explain," "Summertime Blues," "I Can See For Miles" and "My Generation."

The group also played three selections from the new "It's Hard" album. Also featured were "See Me, Feel Me" and "Pinball Wizard" from the "Tommy" LP; "Baba O'Riley," "Behind Blue Eyes" and "Won't Get Fooled Again" from the "Who's Next" album; "Drowned," and "Love Reign O'er Me" from "Quadrophenia" and "Who Are You" from the album of the same name.

To bring any message to 50,000 screaming fans is no small achievement, because even if the band is the focal point of the audience's attention, there is really a tremendous distance between the performer and the observer. But the Who break down that barrier.

They do so with exaggerated gestures and theatrics. Indeed, in the case of the Who, bigger is better.

If they were not larger than life already, lighting and fireworks were used to wake the audience into a frenzy, to bring it closer to the music and its meaning.

The real emotional payoff in this concert came toward the end, during "Won't Get Fooled Again," when the stage lights went off, and then a great flash appeared as Daltrey gave his classic scream. Then came the lyrics that summarize the group's feelings about revolution and reaction: "Meet the new boss, same as the old boss."

This was the Who at their best. Unfortunately, if this really is their last tour, raw emotional power and artistic intensity in rock 'n' roll may be on the way out.

What we are getting instead is music from a group like Loverboy, who played before the Who and said they will be back next year. On the next time around, they will probably bring a new batch of premeditated rock songs with lyrics that are male chauvinistic, if not downright fascistic.

And the Who? They managed to escape after a four-song encore, ending with "Twist and Shout" under the cover of a spectacular fireworks display — and taking with them a gigantic chunk of musical history.