True contending teams don't waste assets like that. If Boston doesn't use it, I think they are signaling they don't actually expect to compete this year.
True contending teams waste assets all the time though.
Golden State just let Payton and Porter walk losing 2 rotiational players worth ~15.3m in assets (based on annual value of new deals) for nothing.
Phoenix seems to letting Ayton walk (though they may get something in return, but it also may be pennies on the dollar).
Milwaukee let Brogdon go for peanuts, didn't use the trade exception they got out of it.
In '21, the Heat let a 7.5m TPE expire.
I don't know if you think Dallas is contending, but they just let a 10.9m exception expire (in hopes of re-signing Brunson).
Clippers have an 8.5m trade exception that expires in less than a week, so let's see if they use it.
Losing assets for nothing (or pennies) certainly happens for contending teams too.