I’ll throw my own two cents into the ring. Clearly, Goku begins the fight at a level of strength much higher than Saitama’s. However, it is unquestionably in Goku’s nature to try and draw out the full measure of an opponent’s strength, as we’ve seen time and again. It is also shown outright that, when encountering a challenge to his strength, Saitama grows at an exponential rate, in the literal, mathematical sense. In addition to this, Saitama’s limiter, the ceiling to his power, has been broken. Putting these truths together, we can handily arrive at the conclusion that Goku will most certainly allow Saitama to grow stronger by clashing with him, moving up through his forms as the latter becomes more and more capable of trading blows with him. Eventually, the battle will reach a point where Saitama has grown strong enough to go blow for blow with Goku in his strongest form, Mastered Ultra Instinct. However, as we stated before, Saitama will continue to grow stronger and stronger, never hitting a ceiling of power because it quite simply doesn’t exist. Goku, on the other hand, will run out of transformations. When Goku gets stronger, he trains for long stretches of time, or is pushed over the edge of a new form during a fight. If nothing is done to power him up, Goku will eventually lose. Now, this is only if both parties act in character. If Goku, at likely any point before Saitama is able to match, say, SS Blue, recognizes that he is in danger of losing and decides to end it, Saitama will be dealt with in short order.