I posted a longer comment above which kinda answers this but without repeating myself too much, Republicans mainly serve to keep Democrats from being able to consolidate power, and Democrats have now shown us what 15 years of a (D) trifecta gets you.

Taxes are higher than ever, cost of living has spiked even more here than other places, tens of billions wasted on CAHSR with nothing to show for it, and no good news on homelessness, poverty, or housing costs either.

Crime has been gradually going down per capita (at least what people have bothered to report), but that's a nationwide trend. If Dems are as good at governing as they claim to be, they ought to have more to show for their efforts than "we had the same results as Texas did."

And the latest Democratic superstars like Mamdani enter the picture, with "government run grocery stores" as his big innovation?

So, my question is why wouldn't one want to deprive these people of more power?

Edit: I'd like to add a reasonable example of what a successful (D) trifecta should have got us, lest you think I'm just some conservative who doesn't want them to accomplish any progressive goals:

As the largest state in the union, California has tremendous power. It could have come up with a 'MediCare for all'-like plan, which would cover all Californians who don't have MediCare. Basically a single public agency that insures all of us, and is run efficiently and transparently and which returns all premiums paid that aren't spent on care as a dividend check back to the taxpayers proportionally to what you paid in. If they accomplished nothing but that in 15 years I'd consider them worth supporting for that alone.

If they couldn't pull that off, successfully building HSR from SF to LA in a decade, by whatever means were necessary, would have been a pretty good accomplishment!

Or finally, if they could have caused enough housing to be built to just keep home prices stable, not even decreasing just stable.

Nothing about this is based in truth.

I won't bother with the rest, but in the past 15 years Dems have only had 2 years of a "trifecta" while republicans have had 4.

Dems have had more presidential terms, the senate has been split, and Republicans have held the House for 11 of the past 15 years. The Supreme Court has been a Republican majority the whole 15 years (with an exception of a tie after Scalia died).

Republicans have the incredible ability to have overwhelming control and agency yet still act like the victims.

EDIT: In response to your edit... you claim Republicans exist solely to keep Democrats in check, and then you list a bunch of things Democrats weren't able to accomplish because they were blocked by Republicans. Your logic is that Dems are bad because they couldn't make certain things happen, but those things were all blocked by Republicans.