Hi Terence & thank you for the lengthy reply. I'll use your numbering to keep it simple...
1) - "It has not replaced either with Photos." That's sort of playing with semantics isn't it? If Apple have "...ceased development of iPhoto and Aperture." And now only produce one photo handling app that appeared at roughly the same time the other two were dropped then I think it's fair to say that the new app has replaced the old even if the new is "...going in a different direction."
2) - Yes, fully understand that you can use Photos without using iCloud. That would never be an option for me with a 1TB+ image library anyway.
3) - I looked at Photos in its earlier incarnations and found it to be a very poor cousin of iPhoto when it came to library management and organisation.
It has been designed more for devotees of the selfie so they can easily track what they shot where & post it to their various SM accounts from multiple devices...
My take anyway.
3) - (part 2) - Incorrect. When you install an OS update or upgrade it automatically installs Photos. No biggie there BUT... when you restart, as you have to after an update/upgrade, one of the first things that happens is that you get a splash-screen telling you your library is being updated by Photos & the only way to stop this update is through a force quit.
This is what I have been onto Apple support about a number of times. I don't have issue with the OS installing or updating an integrated app.
What I have a serious problem with is the OS forcibly updating my library without asking permission & then switching my default image handling to a different app.
It's very dodgy accounting practice on Apples side. You can just see the internal memos "User uptake of Photos app has been very high..."
Yes, it has, because Apple are forcing users to make the update.
4) - When I next update I plan to do that & see if it then keeps trying to forcibly update my library.