1) I suppose that Brady could extend for $5M a year for 2020 and 2021, or even $1M. That would help the cap for those seasons even more than his $15M helps the current cap.
2) Brady has not, and will not demand market, currently $28M a year guaranteed for a quarterback not projected to be nearly as good a QB as Brady over the next 3-4 years.
3) Even at $25M a year, the 2018 cap could be reduced by over $5M and there would be no effect on 2019. So, I don't see how this will hurt in the acquisition of players supporting Brady.
4) After all, if the 2020 or 2021 caps are an issue, there could be a further (and then fake) extension, spreading the dead money over the first years of a young (relatively inexpensive) new quarterback. So, even here, there is no reason that the acquisition of new players to support Brady would be threatened.
5) The bottom line for me is that Brady won't expect market rates. On the other hand, there is little reason to ask him to take a discount of 25% - 50% from the market rate.