I agree with the observation that general trends of smartphone prices increase. If I look back over my last 5 years, yes looks like more each year, I’d guess spending $30-$50 more each year.
But also the phones are doing more. My last phone has 8GB RAM, 256GB storage, all the global LTE bands…
I don’t think it is inevitable you have to pay more, it comes down to personal choice.
I have just watched the review of the Huawei folding phone, Mate X, it looks quite good, once you see folding with screen facing outside of hinge it is so obviously a better idea than outside of screen and the consequence of thicker phone.
What fights folding phones is how tablets seem to be dying. The general trend seems to be in 2019 basically a 6" phone or an iPad and then Chromebook. Items in between seem to be dying.
I currently have a OnePlus 6, a Huawei Mediapad M5 10.8 “phone” (LTE SIM), Lenovo Tab 4 8 Plus, and still my Nexus 7 2013. I think roughly the cost new of them all is less cost than one folding phone. So you must really really like not carrying a bag.