It certainly becomes clear that Obi Wan is simply waiting until Luke shows up. This, of course, turns what was subtext into something absolutely clear and undeniable-- a risky course in storytelling.

You might try to include a beat of Darth overpowering Obi Wan before Luke arrives to show that Obi Wan is not going to win... then his smile at seeing Luke will play more as a "surprise" reveal to the audience. "Ah-- he had a plan!" As it is, it plays like Obi Wan could duke it out, but he mysteriously chooses not to (with a wry smile to prove it's related to Luke being there).

I guess it's not really more subtle either way. But if you put Obi Wan more on the ropes, there will be more dramatic ups and downs in this sequence.