Can I say “on the credit side” in place of “on the positive side” or “on good part”?

I saw the phrase, “on the debit side” and “on good part” as a contrast in the following sentence of New York Time’s (December 17) article titled “Life Goes On, and ON,” that deals with the opportunities and problems of longevity society:

“Is this (extended longevity) a good thing or a bad thing? On the debit side, there’s the ... debit. The old-age safety net is already frayed. According to some estimates, Social Security benefits will run out by 2037; Medicare insurance is guaranteed only through 2024. These projected shortfalls are in part the unintended consequence of the American health fetish. ---

So what’s the good part? Time spent with an elderly parent can offer an opportunity for the resolution of “unfinished business,” a chance to indulge in last-act candor. ... I hear a lot about late-in-life bonding between parent and child."

Obviously “on the debit side” is used in the sense of “on the negative side,” and “good part” for “on the positive side.”

Can I use “credit side,” the antonym of “debit side” instead of “on the positive side / on the good part” in the above sentence? I mean, can I rephrase the last line as “So what’s the credit side? Time spent with an elderly parent can offer an opportunity...”? Or is the word, “credit side” only applicable to bookkeeping?

Yes, certainly.

Though you don't have to, as the writer demonstrates.

The credit/debit side and the good/bad part(s) are both fixed phrases with oppositions built in, and they oppose on the same semantic axis GOOD ⟷ BAD.

On the credit/debit side refers to double-entry bookkeeping, and is part of a metaphor theme that equates having money (credit) with GOOD, and owing money (debt, or debit) with BAD.

So they can be used pretty much interchangeably, as far as meaning is concerned.
In style, however, they're different, because one's metaphoric and the other's not. This may or may not matter to you.

"On the credit side" is a suitable antonym for "on the debit side", but neither of the phrasings "So what’s the credit side?" or "on the good part" are used. "On a positive note" is a common phrase, as is "on the positive side". "On the upside" is so frequently seen as to be cliché.

The terms "credit side" and "debit side" arise from double-entry bookkeeping, where (in paper-based systems) debits were maintained on left-hand pages and credits on right-hand pages, hence "debit side" and "credit side" of the ledger.

Edit: Looking at some ngrams, I see that "on the upside" may be used less than I thought it was. It appears that "on the credit side" was quite popular a century ago, with "on the positive side" being second most popular until after 1960 and most popular since then, not yet overtaken by "on a positive note" and "on the upside".

You can use on the credit/debit side or the good/bad part, as @JohnLawler said, but I wouldn't say "on good part" -- that's just awkward. "On the positive side" and "on the negative side" are another matched pair. If you're just talking about contrast and don't need to call out good/bad, another phrase you can use is "on the other hand" -- e.g. "(bad thing), but on the other hand, (good thing)". The phrase "on the other hand" conveys a change in either direction; if you were talking about a good thing then what's coming next will be bad and vice-versa.