Is it fair to assume Ali would have carried on winning if it wasn't for Vietnam?

Yes. Muhammad Alis last fight before the exile was in March 1967 stopping Zora Folley. He was 25 years of age. He fought again in October 1970 when he was 28 years of age. Those 3 years he would have been peaking. It is too bad. We may have never seen him at the best he could be. He probably could have defeated Frazier if they fought in 1968 or 1969 without a break. Yes he came back strong winning the title back in 1974 at age 32 past his prime by then.

If Ali had been fighting during the period he would have been 25-28, he would have met Frazier earlier and got beaten earlier. I have never agreed with the popular notion that when Ali continued boxing after being reinstated we saw a diminished version of himself. It is clear that he faced much, much superior competition after his return, and to his credit, won more than lost. That includes the two Liston fights which smelled fishy to me then and now.

Like many, many athletes that did serve their country they all would have been much better without this absence. Ali stayed home, but was never the same.

Oh yeah, he was D-virginized after coming out of prison and having to take a fight with Joe Frazier so early on. It takes a while to recover and regain from such a unfairness, stress, and lost.

It is safe to assume he would have continued winning while he was not fighting.

Whether or not he could have stayed motivated and not lost a decision during those years is unknown.

Up till Joe Frazier stopped his train which would have happened sooner had he not retired.

Not just winning, but probably dominating, if it wasn't for his absence.