From a repairability standpoint... Let's just say I've never seen an SSD fail in an iMac and I've been working with hundreds of them over the past few years. They're super reliable and I think Apple realizes this so they cut costs by soldering them on.
That leaves the expansion standpoint. Frankly, this is not a big deal either. Apple doesn't even use standard M.2 drives; they use their own proprietary interface. On top of that, these are iMacs! You can easily plug in cheap external storage, and since the machines typically stay in one place it's not inconvenient like it would be on a Macbook.
I'm still irritated by everything being soldered down, but it's not as big a deal as it was in the past. If Apple bothered to use an industry standard like M.2, it would be a bigger loss, but they don't anyway.
unfortunately I can't share your view here. Had a MBP 15" 2015 SSD fail around second year in without any warnings (usually how it is with ssds in contrast to hdds slowly creeping in errors). To be fair, I also have another Mac with a SSD that hasn't failed over several years now, so while rare not impossible. Should those ever fail out of warranty... I don't even want to think about the costs. Whereas if it was repairable, a quick swap of just the ssd would be all it takes.