A great question. One answered in various ways before now.
Musically, of course, it wouldn't have been the same. It's useless to try and speculate what Marr would have done differently than Street, Nevin, Whyte, Boorer, et al. Although it's probably true that Johnny would have become more experimental, and that Morrissey probably would have followed him down certain paths he has, in his solo career, mostly avoided. Maybe not synths and drum machines, but a little farther-reaching than his music is now.
Lyrically, I think it's pretty well accepted that "Viva Hate" is made up of several songs that would have been Smiths tunes. (This is just my speculation, but "Dial-A-Cliche" was first mentioned by Morrissey in '84 or '85, then called "Father And Son".) A few, like "Break Up The Family" and "Oh Well, I'll Never Learn", seem to have been written after. So "Viva Hate" is a pretty close approximation of what a fifth Smiths studio album would have sounded like.
After "Viva Hate" things definitely changed, and I don't think you can say that Morrissey would have written the same songs. In my opinion, a group dynamic-- not necessarily The Smiths, but a true group dynamic, with at least one other equal creative partner (Marr)-- would have meant that Morrissey's lyrics would have changed less than they have. His unwilling climb into the solo realm changed his outlook on songwriting in a very fundamental way. From "Hand In Glove" to "Life Is A Pigsty" Morrissey has basically put his own life in his music, but in The Smiths there was more of a tendency to present it in a more universal form. The Smiths seem more open-ended to me, more accessible to the imagination than most of Morrissey's solo songs. The illusion of being in a group, regardless of how he actually wrote the songs, gave a different texture to his lyrics.
Again, to pre-empt the inevitable reply saying "Morrissey has ALWAYS written autobiographically!", I'm saying it's a subtle but nevertheless important difference in how Morrissey has presented himself autobiographically. His solo songs have gradually evolved from being universal pop songs (that were clearly his own) to being very clearly about himself, about Morrissey the pop star. Earlier songs like "Interesting Drug", "Everyday Is Like Sunday", and "The Last Of The Famous International Playboys" are a little different in perspective, I think, than "Irish Blood, English Heart", "You Have Killed Me", and "Dear God, Please Help Me". The best comparison is between two "state of the nation" songs like "The Queen Is Dead" and "The World Is Full of Crashing Bores". Both are highly unique to Morrissey, but the former is written by Morrissey in a less specific way. It sounds exactly like Morrissey but you can imagine anyone being the "I" in that song. Not so much 'Bores'.
I really can't imagine Marr putting music to "Irish Blood, English Heart", for instance. But I can easily imagine Morrissey and Marr writing a song about some character or subject involving a mix of things English and Irish. The union of personalities brought out a little more objectivity in Morrissey (and kept Marr away from the disco, fortunately).
But as things stand now I'm glad. I think Morrissey has become exactly the singer and writer he should have become. He became a legend in The Smiths, and now he lives out that role perfectly as a solo artist. Not to denigrate his records, which are wonderful, but in the current musical climate his symbolic importance cannot be overestimated. His music-- besides being good in itself-- is much more important as the ex-singer of The Smiths than it would have been had the band remained together.