If you don’t buy it you will soon envy [a] the one who did
If you don’t buy it you will soon envy [c] the one who
(The Cambridge Grammar of the English Language)
No doubt, [b] is anterior to [a]. But the relation between [c] and [d] is somewhat ambiguous (The book only says [d] is posterior to deictic time [speech time, I think]): [d] might be simultaneous with [c] or posterior to [c] or vice versa, I suspect. Which one is posterior to the other, or does it depend on the context?
My first thought is that this depends on the context, namely, whether this is a product that one buys once and then keeps indefinitely, or if it is something that one buys and consumes in a regular cycle. Consider:
If you don’t buy a house, you will soon envy the person who did.
If you don’t buy diet soft drinks, you will soon envy the person who does.
In the first case, I expect to buy a house and keep it for a long period of time, so the purchase is a one-time event, so the verb should be past tense. In the second case, buying soft drinks is something one would likely do on a regular basis, so a continuous verb makes sense.