I didn't say Brady would carry his current level of play.
Nor do I deny Brady's age. The point is, if he hasn't declined from 35-40, there's good evidence right there to make a reasonable assumption there could be a gentle, slower decline in performance from 40-45 than you might expect otherwise.
The reason for using the next 5 years as a barometer is making a bet on a very good Brady for those years makes the trade make much more sense. After 5 years not just Brady, but most of the team's stars will likely start turning over, if not a little sooner. So having Garoppolo beyond then doesn't matter as much.
Based on the atypically unnoticeable decline from 35-40, I think you can reasonably make an extrapolation that Brady's next 5 years, while almost certainly with an inevitable decline, will still leave him playing at a pretty good level. Which dovetails with my original point- how much better can Garoppolo be? Couple that with uncertainty, plus the high salary Jimmy G would command. Plus you get to put that extra salary to work making the rest of your team better, such as investing in an O-line to protect Brady.