How about instead merging North Dakota, South Dakota, Wyoming and Montana? 739,482 + 853,175 + 584,153 + 1.024M = 3,200,810 which is less than the population of LA but for some stupid reason has 8 Senators.
The constitution? The desire of rural Westerners to not be directly governed by urban voters in Northeastern or Southwestern megacities? The incrementalism of Manifest Destiny?
California also pays more than its share in Federal taxes and gets less in Federal spending. If you look at the Base Closure Commission list, CA lost 28. Mississippi isn't even on the list because they didn't lose any.
Your originalist argument means nothing.
If you want to debate whether the Senate should exist or not, or whether it serves a purpose or not, or whether how senators are allocated should be changed or not, then fine. But that's a whole other thread.
I'd also argue that the Senate serves the important purpose of preventing abuse-of-the-majority, a problem known as far back as Ancient Greek democracy and well-considered by the founders.
Getting rid of the Senate or making Senator allocation population-based with no further structural modifications would simply turn us into the United City-states of America. Which is great... if you live in a city and aren't a regional political minority.
https://en.wikipedia.org/wiki/Federalist_No._10
From this view of the subject it may be concluded that a
pure democracy, by which I mean a society consisting of a
small number of citizens, who assemble and administer the
government in person, can admit of no cure for the mischiefs
of faction.
This is Red State Moocherism. Mississippi exists only on the enforced generosity of the actually productive States.