Who's saying we have to get rid of Rozier? As a guy making only 5 million the next two years combined, we should be keeping him. It's Smart, Thomas, or Bradley that are not signed for 2018-2019 that they should consider trading, since the chances are low we can't re-sign all three.
The thing is, Hayward is already going to be making less if he comes here because Utah can offer more money. I don't know if it's worth it to nickel-and-dime him. Besides, there are only so many minutes to go around. It's nice to have depth, but eventually those bench guys want minutes/more money and they'll leave anyway. You can't keep everyone forever.
Every team eventually pays their stars and uses what's left over to try and construct a bench. You can do that by drafting well (since rookies are on fixed, underpaid deals), signing cheap FAs, or signing vets who are usually close to past their prime willing to take a discount to win.
I don't know what numbers you are looking at, but they're still going to need money to sign a PF. If they sign Hayward chances are Olynyk is gone and I don't think they would want Johnson back anyway. So the 500,000 or so you are asking Hayward to give up is probably not enough. I don't think the Celtics would be wise to have so many rookies, talented or not, being their primary bench options.