Here's the situation: I do a lot of hiking, backpacking and traveling with my partner. I tried carrying a few different point and shoot cameras but couldn't stand them. I tried carrying my old DSLRs but they were too big/heavy/obtrusive with the large body and heavy zoom lenses. So while to fall back to my iPhone 11 Pro I simultaneously took the totally logical step of pre-ordering a Sony A7 IV, which arrived last week. Now, instead of a 28-75, 24-105, or 16-35 I'm seriously considering just two primes: a 24 and a 50. Specifically the new Sony lenses that came out this year (SEL24F28G and SEL50F25G).

Would I be crazy to go with two primes for hiking/backpacking/travel as opposed to a 28-75 (Sony) or 24-70 (Sigma)? Maybe I'd need to add an 85mm. When I shoot with my iPhone I don't pinch around, it's either wide open, 1x or 2x, so I feel like I would not miss the in-between lengths. Please halp me decide!

If 90% of your shots are either at 24 or 50mm, this makes sense. If your shots are about 50/50 at 24 and 50mm, your travelling companions will hate you for it as you're spending ages swapping lenses. To me these lenses make more sense on an A7C, to have a pocketable fullframe camera like the RX1.

There's no Sony 28-75 (yet), but in your case I'd seriously consider the Tamron 28-75 f/2.8 G2 if you don't mind sacrificing a bit on the wide end. The 24-70 f/2.8 options are very bulky (Sony and Sigma) and/or mediocre (Sony and Samyang). Whether you add a wide angle to a 28-75 (e.g Sony 20mm f/1.8 G) or a tele to a 24-70 (e.g Sony 85mm f/1.8) is a tradeoff only you can make: does the prime add enough to your options to warrant the extra weight and bag space needed?

You don't specifically say what camera you were using before, but since you have experience with several zoom, you are a good judge of what zoom lenses would do for you.  And you know that zooms are larger and heavier.  You  are in a perfect position to judge whether your new camera with comparable lenses (zooms vs primes) would be significantly smaller and lighter than your old setup. 

I would guess that the best way would be an single, all-in-one zoom -- but each has its limitations (too many to list here).

As a long-time backpacker, if I were forced to pick a single lens it would be a full-frame 24-200mm.

I don't think you're crazy.

A nifty fifty for Town and general, then the 24mm for landscapes sound reasonable to me. You'd need to get used to "zooming-with-your-feet", but a lot of photographers find the discipline of using prime lenses leads to more interesting photos.

The 40mm is also worth considering - I find 50mm can be a bit narrow for travel.