Hiya, first of all, congratulations! I'm not much help in the details of living in CA, and if you should take out loans. To me, this seems like a really personal decision that is guided by your priorities are. If I had to pay for a PhD, it would only be in a case where it was worth the debt (this includes the long term perspective, because graduate loans usually accrue interest immediately).
What I did want to touch on, however, are the two things that stuck out to me in your post. 1) Needing to respond by Monday and 2) this being your only acceptance.
1) This seems a little suspicious to me. Why would you need to confirm if you're willing to accept such an offer in such a time period? To me, this signals something about the PI will [edit: you] would be working with if you were to accept this offer. People are the nicest when you first meet them and they are trying to make a good impression on you, so you would attend their program. To me, it sounds like it would only get worse. If that's not important to you, then that's fine but I do know that everyone I have spoken to has always said to prioritize your PI e.g. how you get along/if they seem like a good mentor.
2) Quite frankly, it's only the beginning of February. If you have programs you still haven't heard from, I would not count yourself out yet from those programs. While we're waiting, it can seem like things are moving slowly and that denial of admission is a forgone conclusion. But that simply isn't true. Most placed have a timeline of communication of mid Feb to early March. And each program is different with timelines, especially given the increase in applicants over the past few years. If I were to give a suggestion here, I would suggest to the PI you interviewed with that you have more time to consider the offer. To seem generous, you could suggest that given the National Acceptance Deadline in April 15th, a fair timeline for an answer from you would be sometime the week of Feb 14th - perhaps the 18th.
Overall, I hope this was a little helpful. I wish you all the luck in your endeavors. And congrats, again!
