There are probably less than 3 key pieces of a championship team on the Jays right now, that doesn't mean you just trade the rest for what appears to be well below market value.
A 1.5-2 WAR shortstop with 4 years of team control shouldn't be what you give up acquire a guy like Thornton regardless of whether you view him as a key piece or not.
How many players on Boston would you consider to be key pieces of the championship team? Certainly Betts, Sale, Martinez, the closer, Xander, Benintendi...
Name the rest of the key pieces, then look at the rest and ask yourself if you'd trade any one of them for Thornton, then realize what an idiotic statement you just made