You can argue that Morrissey and Marr mined many diverse and vital veins of music and pop culture from the 1950s to the 1980s, transformed what they took into their own unique idiom, and effectively became the final, ultimate pop/rock and roll band which both embodied all their influences and transcended them in a way that ended an entire run of British rock bands starting with The Beatles and The Rolling Stones and moving on to The Kinks, The Who, The Clash, and The Jam. With the charismatic, genderless pop star sensibilities of Morrissey and the diverse musical skills of Marr, which brought folk, blues, r 'n' b, pop, punk, and even a smattering of classical to their sonic pallette, Smiths records make up a living museum of the history of pop/rock music from Elvis to Motown girl groups to the Buzzcocks.
Although other bands made more money and have won greater acclaim, The Smiths have the distinction of more or less ending British rock music, since the only stuff to come out after 1987 was, at best, only a solid imitation of the past (Oasis etc); The Smiths wore their myriad influences on their sleeve but were also distinctly of their time, the Eighties. They were traditional and avant-garde all at once, and the proof of their totalizing, majestic, unconquerable superiority is that, after them, pure, non-hybrid (i.e. not hip-hop influenced) guitar-based rock music became either baroque or mannered (obviously excluding the unmentionable cast of bland characters who make up the safer, contemporary side of music in any era, like Matchbox 20). The Smiths represent the highest peak of the development of rock music-- artistically, if not in actual sales-- encompassing all of the past, perfectly defining their present, and rendering any future for rock and roll (and rock-based pop music) unthinkable and impossible.
My final phase of the argument would be to point out that even Morrissey and Marr recognize this to be true. Morrissey, of course, was already prophesizing the death of rock music while The Smiths were still around, and his solo career, as brilliant as it's been, has largely been backward-looking. He has chosen to emphasize the mythology he created in The Smiths and to stand firm as a legend after the classical Sinatra model. Musically his records sound different than The Smiths in places, but for the most part have not strayed from Marr's basic templates. As for Marr, no better proof of the argument is needed than to note that, with his deep knowledge of and respect for the history of pop music, when he disbanded The Smiths he knew it was all over and almost immediately turned to electronic music. Since "Strangeways" he has contributed traditional guitar music only to musician friends (Billy Bragg) or to artists he considered unique (The The). Joining Modest Mouse looks like it might be a return to form, but mostly his post-Smiths career bears out the thesis above: rock/pop died with its natural artistic peak, The Smiths.
There are holes in this argument but, hey, that's what rhetoric is for.