Belichick has traded Mankins, Chandler Jones etc. He doesn't worry if his sentimental fans think he's crazy, he simply maximizes value and profits where he can. He's not afraid to part with a good player if its best for the franchise and adds value.
You'll notice my post said "if Kouandjio is legit." So its not if he shows a little promise like you say, I'm talking about a scenario where they're confident that Kouandjio can play left tackle
Cordy Glenn playing left tackle is worth more than Cordy Glenn playing right tackle. The same player on the left side is worth more in terms of both money and impact on W/L. Protecting the blindside is a valuable commodity that teams pay for. A team that can't play Glenn at left tackle doesn't benefit from him as much as a team that needs an LT, and so there's a natural opportunity to profit. For a team like the Bills who are desperately seeking a QB and who will need to re-build their core soon, the ability to get younger and cheaper AND add a draft pick is a blessing.