For an average Xperia 10 III owner, in most cases only 1 year old or less, neither the 10 IV nor the 10 V offer anything worth paying several hundred Euros for. They even look the same. The 10 V’s improved camera won’t be of much use just like 10 III’s camera still isn’t (and won’t be until Jolla cracks the Camera2 API). Other than that, I can’t see any other possible benefits of upgrading. I like 10 V’s stereo speaker, but I won’t shell out hundreds of Euros just for that.
So, my opinion is entirely objective, based solely on what I think would be the best for the development of SFOS, and NOT because I need or want to upgrade to either of those models, because I DO NOT.
“Out there already”? “Out there” it was a year ago, and now it is just about to disappear from the stores, just like the 10 III did shortly after 10 IV’s announcement last year. Very soon it’ll be hard to reach first hand.
If so then SFOS will remain an OS for mostly second hand / outlet / closeout devices, as the 10 IV port will - once again - come out only when the 10 IV stops shipping, and so will then the 10 V port. In such case, I’m afraid that we should not expect SFOS to ever expand beyond that nice but niche group of geeks originating from Maemo times.