A 5-year UFA deal would cost more than an 8 year UFA deal, as its always been. A 5 year deal would be age 27-31. Those additional age 32-34 years are entering into decline years, which lowers the price. How the cap is projected to grow is taken into consideration, but signings are primarily based on cap at the time of signing, and there's no way you're going to get Matthews on a 5-year deal for cheaper than an 8-year deal, regardless of what you may think the cap is going to be years from now.
Term is and always has been a trade off of financial and life security vs. maximizing your potential dollars. UFAs consistently choose security. If Matthews wants to take 1 year deals every year to maximize his earnings, that's up to him, but he can't just say pay me long term now as if the cap was 95m because I think it might be in half a decade. Many of the contracts being signed now are ones that will stretch into when the cap is 95m+, but they're still being signed at numbers consistent with the flat cap.