Mallett won't be a FA until AFTER next season. That is before franchise or transition considerations come in. With Brady's reduced salary, I maintain they could pay both $8-10 million a year if Mallett shows enough to be a Aaron Rodgers candidate. Other contenders pay that much for their franchise QBs.
Why would he stay? He could be in the position of inheriting a Super Bowl contender rather than just some perennial loser of a dreg team like the Jets or Jaguars. BB is preparing this team for a series of genuine Super Bowl runs over the next half decade, and that is obvious to any real analyst.
IMHO, He would laugh at your $1.5 million a year offer. IMHO, he might sign a low dollar, one year contract, if and only if, TB retired; and he had a free attempt at a "prove it" year. Staying with a SB contender might strategically over rule the dollars with a dreg team, but I doubt it. He has sat too long, and would want to play.
PS: I could see what Belichick was doing with Cassell in 2008. BB was purposely stacking the deck against him with a scrub Offensive line and future truck driver WRs. He wanted to test Cassell's POISE in adversity, since he hadn't played in college,and couldn't measure it. Cassell never blamed anybody else, and bore the unfairness stoically, while maintaining his cool. He passed in BB eyes, just like proverbial JOB did. Many fans were calling for Cassell's head, and deriding him as hopeless, I wasn't, and applauded BB keeping him and we were both vindicated by his play in 2008.
Could BB doing the similar thing with Mallett, purposely limiting his reps, and not allowing him to show how good he could be, while wasting time with Tim "The Great Embarassment" Tebow? BB might do that to determine Ryan's Patience, and to keep him working hard. That might keep his value lower reducing other offers, too.