Your quote goes against your argument:
It is irrelevant that the voters rather than a legislative body enacted [the initiative], because the voters may no more violate the Constitution by enacting a ballot measure than a legislative body may do so by enacting legislation.
The SC here separated "the voters" and "a legislative body." The Constitution does as well. "A legislature" in a republic is a group of people representing our interests. It is different from a direct democracy where the people rule en masse. Similarly, Art. V. supplies two ways to ratify: legislatures and conventions. The conventions are not some "special" sort of legislature, it is a separate measure. In fact, many feared direct democracy, and saw the wisdom of legislators, including small bodies thereof.
"The people" have various roles in the Constitution including the First Amendment separate from the government itself. They, e.g., have the right to petition. They aren't petitioning themself. They are petiting "the government," including the legislature. The most obvious evidence here is the 10A where "states" and "people" are separatedly listed.
You put a creative argument, but in context surely, it is a bit too clever by half. BTW, the process allows the people to vote on many ballot issues, not just one. An argument can be made this is a problem under Guarantee Clause ("republican form of gov't"), but that's much broader than pointing out that when "legislature" is specifically mentioned legislatures have to act.
It's not a good idea to prophesize and reasoning can be made to allow this measure but it has serious problems.*
-j
* Pre-17A, some states found a way around the "legislature" req. of appointing senators, but there was an important difference:
many states had improvised de facto direct election systems via party primaries and nonbinding popular votes (aka “the Oregon Plan”)
The legislature ["Oregon legislature"] set forth this policy. A decent argument can be made some versions of the plan were unconstitutional, but the 17A came before any serious effort succeeded to do so. In fact, it probably would be different if the CA legislature directly put this "one" issue to the vote here in some special plebscite and held that if the vote was positive, it would put forth the desired policy. It didn't.