> Phones never were open
They certainly were more open than today. It was much easier to fiddle around with the software with smartphones that Nokia used to have, for example. Both iOS and Android were easier in that respect, too. So there's clearly been a regression there. However, if you meant "phones were never as open as PCs", ah yes then I agree.
> As such, for me, this is more of a lost opportunity to progress rather than a regression.
I would agree with the spirit of that, but the problem is that a lot of computers are actually being "dumbed down" so that they are more similar to phones simply because a larger population is used to them. For example most young people might not find it weird if some day MS or Apple make it painfully difficult to install PC/Mac apps from outside their stores.
You're right we are not there yet (yay?), but we seem to be moving towards that.