Wouldn’t something that scratches a regular glass display absolutely shred the plastic screen on a foldable?

Not really for a few reasons - Firstly, most scratches on your phone happen by bumping up against stuff in your pocket, or by getting knocked against stuff when you have it set down. In both of those scenarios, your phone will be folded. You only really unfold it when you need the big screen, and then it gets folded back up again when you're done. So it's more secure than a typical phone screen most of the time. The biggest danger there is getting a piece of debris (like a piece of gravel) on the big screen then closing it, but while I've heard about that, I've never actually had it happen (or even have a close call in that regard, since i don't make a habit of using my phone where gravel is flying around lol).

Also, the folded screen has a "permanent" screen protector on it (I use quotes because it does sometimes come unstuck - but it's easy to replace), so you'd have to cut really deep to even get at the actual screen. Again, I can only speak to my own experience, but in my current phone and my last foldable, I never once scratched the center screen, despite daily use. The front screen however I absolutely have scratched, but that's just due to my own carelessness - like I said, i don't baby this thing.

What I still don’t get at is the normal sized phone when open that closes to a thicker square

I'll be honest, I'm not 100% sure what this means, but I'm going to guess you mean is "When it's closed, it's just a super thick phone with a normal phone screen" - if that's not right, let me know. But yeah you're totally right that it's thicker than a normal phone, but it's not like carrying around a brick or anything, I also have a wallet case on mine, and the size of the fold with a wallet case is less than the size of a "normal phone" + A wallet FWIW. But yeah if you like your phones paper thin, than you're not going to like Foldables, maybe if Samsung ever comes out with that scrollable phone they've been teasing for years now they can make that one thinner.

that you can barely use when not opened

I guess this varies from person to person, but I use my phone all the time folded, and it works great. The folding screen is too big to operate with one hand comfortably, so when I'm just doing "normal" phone stuff, i generally always stick to the front display, and I've never had any issues using it. I actually prefer the "skinny" form of the screen to your typical wide modern smart phone for a lot of stuff. On my wifes iphone I find that i can't actually reach the whole screen when using it one-handed, while on my phone i can comfortably reach every part of the front screen while one-handing it (part of that ironically is, I think, due to the slightly thicker size.

Now I don't wanna sound like a shill here, so I'll throw in a bit about what i don't like about the phone

Dust: I do a fair bit of woodworking, and I found with my Z Fold 3, that overtime the dust buildup in the hinge caused it to not open all the way. With my Fold 4, I either keep it out of the workshop while I'm working, or put it in a ziplock before going in. So far, I haven't noticed any buildup on my 4 taking this precaution, so I think it's only really an issue in the sorts of extremely dusty cases you get in woodworking and things like that, but still something to keep in mind.

Battery: Definitely my biggest complaint by far, under "normal" use - I can make it through the day no problem, but if I'm going to be doing anything that falls under the umbrella of "heavy use" (which means anything that makes the foldable screen worth using) then battery life drops off pretty quick. Not a huge issue for me, as I work from home, so I keep mine topped up at 85% most of the time anyways, but if you need to be away from chargers a lot, then this isn't going to be a good fit for you.

Ultimately whether it's invaluable or a "gimmick" depends on the person in question. For me, it's the perfect compromise between a smart phone and a tablet for someone who has use-cases for both, but can't justify lugging a tablet around everywhere - sounds like it isn't for you, and that's alright