Given that many of the roster spots are taken by players under contract, many/most AAVs are already known. That leaves only a few assumptions to make. So, I wouldn't characterize it as a "pretend" roster or exercise.
* Note - I have created many spreadsheets in the past and posted them before. Some of the spreadsheets show a roster that meets the cap, and some don't. If you've been in here and are paying attention, you would have noticed that already I'm sure.
So, I've already done this, and will of course, continue to do this.
In many cases, I take the ideas generated by other posters, and then plug them into the spreadsheet. I take different hypotheses that people make in here, and then run the numbers to reveal whatever they show. Whatever the numbers are, they are the numbers. You could say just let the numbers speak for themselves.
Running the numbers is informative and it is a different approach that sets aside emotions and preconceived ideas that some people are more prone to.
In general, when more expensive players are added to the known salaries (good players already under contract), the more likely it is that the total will exceed the cap when the cap is tight. And, next year, given that that the cap will likely be $83.5 m, it will definitely be tight.
I think you would find the exercise quite useful. It could be much better than making blanket statements (opinions).