For a while I upgraded my dSLR body every 18 months to 2 years. Then I invested in a Hasselblad H3DII. This is still giving excellent results well suited to my main business, 8 years on. It cost a not so small fortune, but by now has paid for itself at least 50 times over.
I bought an A7RII because it covers the two shooting scenarios that the Hasselblad really didn't - available light shooting in low light levels, and portability for lugging up mountains. It's paid for itself, expensive lenses included, in the couple of years I've had it.
But yes, stills cameras are a pretty mature technology at this point. There's room for a shakeup of how we control them in my opinion (we're still controlling them like they are film cameras, rather than fully embracing the digital aspects) but I don't see anything massively revolutionary coming along now we have 50-ish megapixels, on a stabilised platform, with a plethora of lens options from ultra-sharp f/1.4 mega modern lenses through to crappy old Russian flare monsters on the front.
I will be buying an A7RIII because I need a backup body for the Sony system and because it addresses the biggest issues I've had with the A7RII (autofocus, especially controlling focus points and eye focus, and crappy battery life).
In the same time I've gone through iteration after iteration of cameras primarily for video, because that's not such a mature technology. I've got phones and stabilisers and GoPros and Pansonics and Canons and all manner of kit, but I'm finally just about settled with a RED Scarlet for "big" work mostly on static heavy tripod and the A7RII on a Zhiyun Crane for anything more nimble. The touchscreen of the A7RIII will hopefully make this use case a lot easier, and I'll be down to just four cameras in day to day use. I'm planning on a mass sell-off of second hand kit if all goes well, actually, along with a bunch of lighting now LED tech has settled down. Usable ISO 1600 + Arri Skypanels = my video lighting needs sorted.
So I think it is all down to your use cases, and whether the tech has quite caught up yet. I mostly sell pics online, just moving into print sales but those are only up to 24" x 36" so 42 megapixels is plenty for me.
But I'm sure there are plenty of use cases where the tech isn't all it could be just yet, and so you might still be in the replace bodies every 2 years cycle.
Cheers, Hywel