Your points are exactly what frustrates me the most about what Apple is doing. I felt like my iPhone 6 was working great, and there was really no need to upgrade the hardware. iOS11 gets installed, and suddenly it's a laggy mess. I'm looking at my $1200 iPhone X, and I'm foreseeing a need to upgrade in 3 years, and I doubt Apple will make the iPhone at that time any less expensive. I can choose to stop updating the OS now, at the expense of security, features, and app compatibility. It's not as hard a pill to swallow when you have to upgrade a $600 phone every 3 years, but a phone twice that cost every 3 years makes my wallet weep.
And I'm typing this on my MacBook which is now 5 years old on High Sierra. I am afraid to jinx myself, but it's running relatively smoothly. I do cringe each time I hit the OS update button, expecting it to be the last time my Mac runs smoothly.