You are quite correct, of course a 2007 iMac can still be usable. I do kinda sorta get some of the people - when you pay a lot of money, you have some expectations, and UI lag is probably not what you expect from the most expensive iMac on offer. However, while I do *understand* these expectations, the *reactions* I do not understand. It's like buying a great, fast, beautiful (and expensive) car and finding out that the audio system inside is not quite what you would expect - and it's not replaceable. Sure, you can be disappointed with that aspect - but will you return the entire car (that is otherwise incredible), enraged - just because of the audio inside? It's like - "I didn't buy the car for driving, I bought it for listening to music".
And that's where your comment fits perfectly: claims that occasional lag makes the computer unusable are just wrong. I don't care what you do, and what your job is, if moving sliders in Lightroom at a slightly lower speed than you expected prevents you from working, then you're either overreacting, or are downright incapable.
So, thank you for your post, I agree completely.
When you look at how Apple has marketed their products over the years, one expects it to work as advertised. I see no issue in wanting expensive products to work as the manufacturer says it does. I could never criticize someone for that.